A2DP sink audio focus

Resolved a bug where BT media fails to pause and subsequently resume during
a transient audio focus loss as is seen during voice recognition.
Resolved a bug where BT media may not resume after a phone call due to an
audio focus race condition.
Resolved a bug where BT phone notifications could interrupt and disrupt
an ongoing media session such as radio or local media player.

Bug: 34853256
Bug: 36529639
Bug: 37288772
Test: runtest bluetooth -c
com.android.bluetooth.a2dpsink.A2dpSinkStreamHandlerTest
Change-Id: I5261d24fd7bbe49bf61c48fdf2c9ae86934dcd3d