Audio focus: duck new players during the ducked phase

Keep a list of all UIDs whose players should be ducked.
  Whenever a new player is created or started, and their
  UID matches one in the list, add them to the ducked
  players.
The bug observed was when Play Music transitioned from one
  song to another while music should have been ducked: the
  new song wasn't unducked because it wasn't in the list
  of players to unduck, as it wasn't active when the ducking
  occured.

Test: request focus while Play Music transitions from one song to another
Bug: 36232742
Bug: 30258418
Change-Id: If8bd47128e6177731406f39e77d049b0d6f34f5d