Cleanup the CarAudioService

What's been cleaned:
- Focus management
- Media mute

Note also
- Tests on focus and mute/unmute are also removed
- currents.txt updated via "make update-car-api"

Test: lunch bat_land-userdebug && m -j // Verify in kitchensink

Change-Id: Iae981177e04b44267650e246f3d7792b5e143f01
17 files changed