A2DP Sink: Focus gain while transient loss

While Bluetooth is under a transient focus loss if the user presses the
play button, Bluetooth should attempt to regain audio focus rather than
just wait passive for the transient holder to release it.

Bug: 143916073
Test: atest A2dpSinkStreamHandlerTest
Change-Id: I7b958a9ff06c9f1fcb9383a3a40ca147f4ad9399