Forward in iPhone leads to play state being presisted as STOP.

When we press forward in iPhone (via AVRCP pass through or iPhone UI)
while connected over AVRCP we are resetting the state and expecting
AVRCP to give us a new play state when next song resumes. Turns out that
iPhone always keeps it state to be Playing.

The fix is to keep the mRemoteMediaPlayers a replica of what AVRCP says
so that when song resumes (from A2dpSinkService via startAvrcpUpdates)
we can restore the playing state.

Tested on iPhone and regression tested on Nexus 5X.

Bug: b/27768620
Change-Id: I8caf56072fa5743f28e1680860c161d3cd75b83c