Detach the existing views instead of removing them in the onLayoutChildren for better performance (#178)

When a view holder is retrieved from the detached state (scrap) is doesn't go
through the Adapter, thus it should be more optimized than re-creating fro the
Adapter.

- Flex line has multiple flex itmes.
- Flex grow attributes for some items are set to positive, the rest of the
  items have the default value
- The item which hasn't the positive flex grow attribute has the largest cross
  size

Fix the issue an item being edited is changed before the user clicks the ok button in the demo app. (#176)

RecyclerView implementation is selected.

When scrolling to top, the anchor position of the first visible position was
wrong when flex direction is set to row_reverse.
This PR fixes that issue.

- Preserve the flex items state across a configuration change in the demo app
- Throw an Exception when order or alignContent which aren't supported by
  FlexboxLayoutManager are changed (setXXXX is invoked)
- Clear the existing flex lines and views if an incompatible, in which prior views
  states or flex lines states can't be shared, attribute change happens
  (e.g. flex direction is changd from row to column)

This PR fixes the view alignment along the cross axis when RV scrolls toward
the start position and view's sizes along the cross axis in a same line are
not the same.

There was an issue that the anchor position was based on the last (or first
when scrolling to top) item in
the last (or first) visible flex line when scrolling to bottom (top).
But the anchor position should be based on the
flex line position (view which has the maximum bottom position in the same
flex line)

Fix the stretching views calculation when the calculation happens from the middle of the items (#164)

When the calculation of stretching views happen from the middle of the items, the flex line being processed needs to be the corresponding one depending on the index of the view.
This PR correctly handles the flex line.

E.g. A new item is added on the condition that RecyclerView is scrolled to the
bottom and align items is set to stretch

For the case where:
- align items attribute is set to stretch
OR
- align self attribute for individual flex item is set to stretch

Becauase the views are laid out as a unit of a flex line, there might be
inconsistency if views are recycled individually.

The crash happend because invalid next position is set in the fill method when the RecyclerView scrolls to top with the scroll speed too fast enough to put two or more flex lines in the fill method.

In the RecyclerView, only the visible part of the RecyclerView is necessary
to be rendered. Thus, not all the flex items need to be measured at first.
This PR makes the flex lines to be calculated only the visible part.

Also this PR removes the support for the order attribute from the FlexboxLayoutManger because calculating the order requires all view to be inflated at least once, which may lead to bad performance if there is a large number of items in the adapter.

Add support for the flex direction is set to row_reverse and the support for the RTL environment. (#149)

Add support for the following case:
- flex direction attribute is set to row_reverse
- Language is set to one of the RTL languages

* Add support for the flex grow attribute for the FlexboxLayoutManager.

This CL introduces a caches for measured width/height measure specs.
Because in during the layout phase for the FlexboxLayoutManager,
a view retrieved from the Recycler may be re-created from the last maesure,
making the measured width/height set to 0 even if each child is at least
measured once in the FlexboxHelper.
So when the flex container is the FlexboxLayoutManager, the FlexboxHelper
stores each measured width/height specs in the cache.

FlexContainer and FlexItem interfaces are created to treat the two flex
container implementations (FlexboxLayout and FlexboxLayoutManager) in a
seamless manner but the visibility of the interfaces were marked as pubic
despite that two interfaces were not intended to be used from external
developers.

This PR:
- Makes the visibility of the two interfaces as package private
- Moves some of the demo app files to the same package as the interfaces
  so that demo app can access the interfaces.
- Change the item type of the FlexItemAdapter from
  FlexItem to RecyclerView.LayoutParams since abstraction layer wasn't needed,
  this class was used in only the RecyclerView path

This PR extracts the calculation to determine the cross size of the flex container from FlexboxLayout to FlexboxHelper to reuse it from FlexboxLayoutManager.

For reusing the logic from both FlexboxLayout and FlexboxLayoutManager, extract
the logic to determine the main size of the flex lines. (Expand or shrink the
flex lines depending on the flex grow/flex shrink attributes)

Implement the rest of onLayoutChildren.

This PR implements the basic layout and scrolling/recycling features for only
the case where flexDirection = ROW and flexWrap = WRAP and the vertically
scrolling.

Extract the common logic to calculate the initial flex lines from the
FlexboxLayout to FlexboxHelper to re-use it from the FlexboxLayout and the
FlexboxLayoutManager.

This PR adds a Fragment that contains RecyclerView as its playground implementation.
And from the RadioGroup in the navigation view, two implementations can be switched (ViewGroup and RecyclerView).

The FlexboxLayoutManager doesn't have any layout/measurement logic at all. Thus when the Fragment for RecyclerView is chosen, no views are displayed regardless of the items in the adapter.

Refactor the demo app to make it easy to reuse the common part across the playground that uses FlexboxLayout and RecyclerView.LayoutManager.

To make it easy to reuse the common part across the playground that uses FlexboxLayout and RecyclerView.LayoutManager by extracting the common logic from the MainActivity into other classes.

Follow up CL is in Progress to add a Fragment for RecyclerView.LayoutManager.

Adds the methods that FlexboxLayout and FlexboxLayoutManager have in common
in calculating the flexbox layout.

Change-Id: Ib0e0159cf0ea2cdc4f9c38020f4b34ec5eb2e537

Extract the common behavior of a flex item as a dedicated interface to treat
the known subclasses (such as FlexboxLayout.LayoutParams and
FlexboxLayoutManager.LayoutParams) equally from the flexbox calculation point
of view.

Remove the FlexItem class in the demo app that was used as a Parcelable. Now FlexboxLayout.LayoutParams and FlexboxLayoutManager.LayoutParams both implement Parcelable.

* Extract the common part of the flexbox logic to reuse it from the
FlexboxLayaoutManager.

Change-Id: I92c4b2971badcde2d9afcf63d1785137def0abcb

* Address review comments from yaraki

Change-Id: Ibbe585fa470fbd4426db64d08278051fddacc85e

* Address the second round of review from yaraki and yigit.

Change-Id: I1e532edacba274c6374deb15b868b0cc7b6d83a6

This PR fixes the incorrect alignment where:
- justifyContent is set to either or space_between or space_around
- Views whose visibitilies are gone are included in a flex line

Also use the number of item count, which the number of gone items are filtered
where appropriate.

Fixes #162

This PR fixes the case where:
  - layout_width is set to wrap_content for the FlexboxLayout
  - End (or start) margin is set to a child
  - The child which the has the end (start) margin has the largest width in the
    same flex line (or only a single child exists)
  - The child has a positive layout_flexGrow (or layout_flexShrink) attribute set

If these conditions were met, the width of the FlexboxLayout didn't take the bottom
margin on the child into account.

Fixes #154