A2DP Sink focus handling

No need to continue setting the setWillPauseWhenDucked flag.  The audio
policy should handle it.

Test: runtest bluetooth -c com.android.bluetooth.a2dpsink.A2dpSinkStreamHandlerTest
Bug: 122710846

Change-Id: I26496f0ba8c13626f8f4d3baa20a31e466e93085