increase the bluetooth task priority when start a2dp.

change the BT task priority based on audio play state.
increase the BT task priority to ANDROID_PRIORITY_URGENT_AUDIO
,when start ad2p audio playing.
to better prevent CPU premption by other process/task(UI).
restore the BT task priority when stop a2dp audio playing.

bug:12082841
Change-Id: I34e8344cffea87f68987149c820cd3e84a4196d1