A2DP sink audio focus

Update AudioFocusRequest to receive notification on
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK.

Bug: 38389934
Test: stream music over Bluetooth and play Nav guidance, observe music
is ducked.

Change-Id: I1b4d33fd63cc6033c06bf24ac0f9a41aeeaf292d
Merged-In: Id0403116019e46191508ed9a4f6f627467db1726
(cherry picked from commit c1a28a63)