AudioService: manage hearing aid volume for calls

Add a mechanism to support audio devices that behave like "absolute
  volume" in AVRCP, but that support multiple use cases based on
  the audio mode.
Treat hearing aid as such a device:
  any change in audio mode will cause the volume of the corresponding
  dominent use case to be applied.
Fix a typo in SCO client death that caused a class cast issue.

Bug: 131091438
Bug: 129163231
Test: set hearing aid volume to X for media, take phone call and set volume to Y,
end call, verify volume is X for media.

Change-Id: Id778f69b98825c83b7137917bd81e973298ab8bb