Refactor SoundPool for appOps handling through PlayerBase

Modified the signature of the abstract volume methods so
  it is clear at the subclass level whether the volume
  command is for a mute or a volume control.
  Changed the implementations in the subclasses
  accordingly.
Removed appOps handling inside SoundPool and made it
  inherit from PlayerBase.
Moved handling of the camera sound restriction from
  SoundPool to PlayerBase.
Added support in SoundPool native implementation for
  muting, as each player has its own volume.

Test: play a long file with SoundPool and enter DnD mode

Bug: 30955183
Bug: 28249605

Change-Id: I0fcd7480f9a455c06aa4f7092486f5c65bc9d7db