Update androidx version

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

* 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.

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`

Implement space_evenly as one of the values for Justify Content.

To not to force the dependent projects to use the specific version of
the support libraries (or explicitly exclude them)

- Able to put MinWidth, MinHeight, MaxWidth and MaxHeight as either of
  one line [MinWidth] [MinHeight] [MaxWidth] [MaxHeight]
  OR
  Two line such as
  [MinWidth] [MinHeight]
  [MaxWidth] [MaxHeight]
  Instead of being placed as follows
  [MinWidth] [MinHeight] [MaxWidth]
  [MaxHeight]

- Give WrapBefore smaller space so that it's more likely that AlignSelf
  spinner is placed in the same line

By chanigng the View type as nullable in onItemSelected

Also fixes:

- Some Kotlin errors in the demo app.
- Replace the deprecated compile dependency syntax to api or implementation in the build.gradle files.

Migrate to CircleCI 2.0

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