Volume policy updates.

 - Make volume policy settable by the volume UI instead
   of hardcoded in AudioService.
 - Add status bar icon for silent mode.
 - Limit unmute-on-volume-adjust behavior to tvs.
 - Ensure all changes to device volume are sent through
   setIndex so no change events are missed.

Bug: 19260237
Change-Id: Iea070a7a6f90ff620e39629f2da3f33f87223d72