VideoView: option for audio focus, support for AudioAttributes

Add API for VideoView to select whether it uses audio focus during
  playback, and how.
Add support for AudioAttributes

Test: cts-tradefed run cts -m CtsWidgetTestCases -t android.widget.cts.VideoViewTest
Bug 30955183
Bug 30258418

Change-Id: I581d32c79c78b8197ded2319e0d5bfdc35b93c5e