AudioService: implement TV system volume

Implement volume policy for TV devices:
- No ringer mode: never silent
- All stream volumes are synchronized: changing any
stream volume will change the volume for all sources.
- Volume is maintained per output device.
- If a CEC capable HDMI sink is connected, digital volume
is maxed out and volume controls are sent to HDMI sink.

Bug: 15759753.

Change-Id: Ic4c38c749ef0440def9635a1669068ccef02a323