level < 17.
Because the FlexboxHelper expected
ViewGroup.MarginLayoutParams#getMarginStart (getMarginEnd) methods when
calculating the wrap condition.

No devices are available on API level < 17  on Firebase Test Lab. So
manually ran the tests on an emulator with API level 16.

* Convert tests in `demo-playground`

* Convert tests in `flexbox`

After this change, minSdkVersion is going to be 14 since the support library 26 isn't able to set the minSdkVersion lower than 14.

Also findViewById now requires type inference. Modified the code that uses findViewById.

Fix the types of the arguments that may be passed as null;

Address review comments from magiepooh https://github.com/google/flexbox-layout/pull/329
after the original PR is merged.

Kotlinize the playground demo app

* Convert Java files to Kotlin

* Update following @thagikura's comments

Remove the part mentioning a dedicated RecyclerView page

For missing description for RecyclerView

For a stable release including RecyclerView integration:
- Bump the library version.
- Update the README

Basically, it apply the same fix as another PR
https://github.com/google/flexbox-layout/pull/304
to the method that stretches views.

- flexDirection is column or column_reverse
- layout direction is RTL (e.g. language is set to RTL language)

If these conditions are met, the scroll direction is considered as
opposite since the start and end direction should be considered as
oppsite from the normal case.

This PR makes it available to let RecyclerView with FlexboxLayoutManager
scroll along the main axis if the width of the RecyclerView is larger
than its parent.
(E.g. When the RecyclerView width is set to 800dp while the parent width
is 500dp)

This wasn't possible even wrap the RecyclerView with
HorizontalScrollView because in that case, the widthMeasureSpec is sent
as <width_of_parent> | View.MeasureSpec.UNSPECIFIED, thus the wrapping
doesn't happen regardless of the size of the flex line being processed.

Because:
- There is an issue that invisble flex line's position is not updated
with FlexboxLayoutManager.
- The same information can be retrieved from an individual view.

- Reuse the same instance for the FlexLinesResult instead of creating a
new instance every time in the calculation of flex lines.

- Increase the default allowing error in tests to reduce the false positive
  failures

conditions that

- Adapter has multiple viewTypes (1, 2. Assuming 2 is the special viewtype)
- A new item is inserted to the adpater before the view which has the
  special viewType
- At the time item is inserted, the special view is not visible.

In that case, the index of the view which has the special viewType is
shifted by the number of items inserted before the position.

This is a workaround to fix the continuous ci failures.

Fixes the issue that only single line was displayed with nested RecyclerViews
where the inner RecycerView's adapter is FlexboxLayoutManager with FlexWrap is
set to wrap.

This was caused because height 0 was passed from the parent RecyclerView
if the layout_height is specified as wrap_content, thus
FlexboxLayoutManager didn't calculate the entire items.

This fixes #290

- the first item's visibility is gone
- the rest of the items (e.g. three in total) have layout_flexGrow attribute set.
- the second view has layout_wrapBefore attribute set (no items are
added to the flex line where the gone view is included)

In that case the index of the views after the gone view is not correctly
counted in computing flexGrow and flexShrink.

Fixes the case where the first view's visibility is gone and the second view is in the second line. (#300)

In that case, the position of the second view is misplaced.

Fixes #283

The issue was caused because FlexboxLayoutManager returned a scrapped
view holder when it should be returned through the RecyclerView (it's
also returned from the scrapped views, but additional procedures are
processed, such as passing the payload objects if any,
if returned through the RecyclerView)

when the user scrolls the RecyclerView.

This was because of the reasons that:
- FlexboxItemDecoration#isFirstItemInLine wasn't working correctly (it
expected the flex lines only being calculated in the
FlexboxHelper#calculateFlexLines, not expecting the already calculated
flex lines).

- In the layoutFlexLine method in FlexboxLayoutManager,
calculateItemDecorationsForChild needed to be called before added to the
RecyclerView because the view man be created in that method, in that
case calcurated decoration's inset values are not set at the time of
creation.

Fix item's crosssize calculate bug

- compute(Horizontal|Vertical)ScrollExtent
- compute(Horizontal|Vertical)ScrollOffset
- compute(Horizontal|Vertical)ScrollRange

You can enable the scroll bar by adding the attribute to the RecyclerView.

E.g.
```xml
    <android.support.v7.widget.RecyclerView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/recyclerview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="horizontal"
```

Note that the set of methods estimate the non-visible part of the flex container by the number of visible/non-visible items. So the returned values are not exact value of the container.
To calculate the exact value, we have to compute all the flex lines including even the non-visible part, which isn't practical since it may take considerable amount of time.

Fixes #249.

The same SmoothScroller as other built-in LayoutMangers can be used.

A dirty flag is used to indicate whether the flex line that hods a dirty
flag needs to be recomputed.

Fixes #262

Remove unused methods that should have been removed during rebasing
master.

Add a set of methods that return an adapter position of the
first/last visible item.