VV2 and MCV2: Override onLayout of ViewGroup and remove MCV2.isShowing()

- Overriding onLayout() is needed as VideoView2 and MediaControlView2
extends ViewGroup.
- Removes MediaControlView2's isShowing() method, since it looks
redundant API.

Test: build, VideoViewTest app
Change-Id: If4608ab56ca3f3883cb674c857f7e53e2dd0e73e