AudioFocusRequest: add ability to force ducking for a11y

New API to specify an accessbility service wants to force ducking
  in its focus request, regardless of the framework's evaluation
  of whether or not it should duck the other players.
Use of this API is restricted to requests coming from services
  bound to the A11y service, or root

Bug: 62194333
Test: play a podcast in GPM and "adb shell claf --gain 3 --usage 11 --forceDuck"

Change-Id: I85f26afa14fe233a3b870f86c9084613203494b8