Fix compiler warnings

Also bumps the Kotlin version to 1.3.61

* Official Gradle Wrapper Validation GitHub Action

See: https://github.com/gradle/wrapper-validation-action

* Add `pull_request` to list of triggers

The method doesn't exist in API level 21.

CompoundButtons such as Checkboxes, when are allowed to shrink (flexShrink = 1f), during the recalculation can end up being shrunk more than what I would consider their "minimum size" since the negative space is calculated for all FlexItems in the shrinkFlexItems() method.

In this PR I try to prevent these components to get completely shrunk by considering the minimum size of their button drawable. In this way, the drawable size will determine the minimum size of the widget unless a minWidth or minHeight are specified by the user.

In this PR I am trying to address an issue that occurs when the calculation of flexlines exceeds the permitted size of their container and some items can be shrunk.

After successfully calculating the number of flexline the FlexboxLayout proceeds its measurement by determining the main size through the determineMainSize() method. In certain situations, the width of some flexlines might be bigger than the size of their container. In this cases, we need to restrict their calculated width by retriggering a second layout calculation and enforcing their width as the one of their containers.

Fix the sentence

Update README

- AGP to 3.5.2
- RecyclerView to 1.1.0

This PR reduces the number of measure calls for the children of the
Flexbox.
If any items in a FlexLine don't have the flexGrow or flexShrink
attributes set, the children in the FlexLine don't have to be measured
in the FlexboxHelper#determineMainSize method.

Similarly, if the AlignItems attribute for the Flexbox is not equal to
AlignItems.STRETCH nor any items in a FlexLine don't have the
AlignSelf.STRETCH attributes, the items in the FlexLine don't have to be
measured in the FlexboxHelpers#stretchViews method.

Also this PR changes the default values of FlexboxLayout as follows:
- mAlignItems: STRETCH -> FLEX_START
- mAlignContent: STRETCH -> FLEX_START

* Update the target sdk version and library versions

* Fixed issue 474, canScrollHorizontally() throws NPE if the RecyclerView is not attached to the Window

Fixes the wrong positioning

On the condition flexWrap = No wrap and justifyContent = center.
This happened because when flexWrap = no wrap, the anchor position
retrieved from the existing view was wrong. Because the OrientationHelper
is changed to perpendicularly when flexWrap is nowrap.

Update LICENSE with proper year and name

Copy from https://github.com/google/fastboot-mobile/blob/master/LICENSE

Added the notice that starting from 1.1.0, the library is expected to use with AndroidX.

Merge the androidx to master

Now Android Studio 3.2 and the androidx 1.0.0 versions are stable.
Merging the androidx features to the master is ready.

* Convert the project to androidx.

Here are the steps I did for the conversion.

Used Android Studio 3.2 canary 17

Auto conversion to androdx
1. Changed the targetSdkVersion and compileSdkVersion to 28
2. From Android Studio, navigated to "Refactor -> Refactor to AndroidX"

Manual changes
1. Trimmed the leading package name (e.g. androidx.recyclerview.widget.RecyclerView -> RecyclerView in files where it's already imported)
2. Added "androidx.test:rules:1.1.0-alpha3" for a dependency
   (com.android.support.test:runner was there before the conversion, but some classes are now provided by a different dependency like "ActivityTestRule")
3. In build.gradle files, defined duplicate versions through variables.
   (e.g. 1.0.0-alpha3 for the androidxCoreVersion)
4. Handled API changes
  - LayoutManager.getChildAt is now nullable
5. Optimized import order

These issues need be addressed by the conversion tool. I'll file these
as issues.
1. double quotations were replaced by single quotations in the dependencies in build.gradle. It's going to be an issue if the versions are defined through variables.
2. Unneeded leading package names.
    e.g.
val navigationView: com.google.android.material.navigation.NavigationView = findViewById(R.id.nav_view)
  Should be
val navigationView: NavigationView = findViewById(R.id.nav_view)
3. Add a missing dependency automatically? Like androidx.test.rules.

Remove the unneeded guard condition in the updateDirtyPosition.
The mPendingScrollPosition (the position that is considered as the
starting point when the next layout starts) needed to be updated
regardless of the if statement of comparing
firstVisiblePosition and the positionStart

Fixes #439

To make it easier to import the scheme.
Also modify the instructions to apply the scheme.

flexbox-layout has been considered as stable for a while, but from this
on, it starts to follow the semantic versioning.

Add maxLine support from the XML

FlexboxLayoutManager in the binary even if it's not used.

Fixes 74417025 internally.

Fixes #332

Removes buildToolsVersions because an appropriate version is picked

As of gradle 3.0 `compile` is deprecated and should be replace by `implementation`