Audio Focus for A2DP.

When AVRCP is available we provide the following focus management on
SINK role:
a) Both AVRCP CT and AVRCP TG can notify the A2DP SNK about play status
change. A2DP SNK will only play when either CT or TG says play.
b) If AVRCP does not exist or neither of CT or TG have explicitly said
play (pass through or notification) then we simply throw away the data.

TODO:
Implement additional policy where we take the data from the A2DP SRC
irrespective of whether AVRCP exists or not. This should be implemented
as a separate streaming state machine and we should be able to flip the
state machine at run time (via settings app).

Bug: b/26928143

Change-Id: Ie5fab15ba4cdd5738c59fa646ca0824770489ec5