Add Media Playing API

These methods permit an activity to play or continue playing media
behind a translucent activity above it. Particularly the home
activity in Android TV. Methods exist to notify the upper activity
when playing starts or stops and for notifying the playing activity
when to stop playing and release its resources.

Methods are called when either activity's state changes or new
activities are launched.

Fixes bug 14469711.

Change-Id: I7ba10c5a4683504931cffa228488f5281e5bbf86