AudioService: unmute media on connection of new audio accessory

When plugging in a new audio device:
  - if the device is considered an external accessory
  - and media volume is muted
  - and the media volume is non-zero for this device
  - and it is used for media playback
  then unmute STREAM_MUSIC.
Log when AVRCP absolute volume is supported.

Test: mute media volume with icon, insert headset then play music, music should not be muted
Bug: 30440012

Change-Id: I102f26c435fa5cbd1e1bc4c14f51629d3cc51fb3