Audio service: always unduck focus requester when granted

When an app is requesting focus, it might
  have been in the focus stack before, and ducked. Always
  unduck the associated players when the app requests audio
  focus and it is granted.
When an app gains focus from an another abandoning focus,
  do not make unducking conditional to having a focus
  dispatcher (== app has a focus listener).

Test: test app made to duck, re-request focus and verify not ducked
Bug: 37433811
Change-Id: I7e5b29c9148c923e7510957b971e28b6139d6156