[android] Refactor layer/source ownership

[ivovandongen]

Fixes #6995

Should be ported to the 4.2.0 release branch as well.

cc @jfirebaugh @tobrun

[mention-bot]

@ivovandongen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bleege, @tobrun and @incanus to be potential reviewers.

[tobrun]

I was able to make it crash with:

java.lang.NullPointerException: Attempt to invoke virtual method 'com.mapbox.mapboxsdk.style.sources.Source com.mapbox.mapboxsdk.maps.NativeMapView.getSource(java.lang.String)' on a null object reference
at com.mapbox.mapboxsdk.maps.MapboxMap.getSourceAs(MapboxMap.java:159)
at com.mapbox.mapboxsdk.testapp.activity.style.RuntimeStyleActivity$3.run(RuntimeStyleActivity.java:355)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5515)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:764)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:654)

Which points to:

 GeoJsonSource source = mapboxMap.getSourceAs("dynamic-park-source");

Seeing this line in this PR makes me think we need to update that example code as well?

update: isn't reproducible on demand.

[ivovandongen]

@tobrun This is a separate issue where the animation is not killed properly on closing the Activity.