Add back the background when we show the action bar

This change fix the following UI issues:

1. We add a black backgorund when ActionBar shows and remove it
   when we hide the action bar.
2. Make the postion of video stable when opening
3. Prevent glitching when opening a video
4. Make it really full-screen.
5. Hide ControlOverlay at begining.

bug:6519765
bug:6491674

Change-Id: I3ab033642df2c4a158b99385b02e3e967eebeabd