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: Ie5a5e13977fafcb4a926fe602efec15a5b1cce83