Make Bluetooth audio threads use RT scheduling

Bluetooth threads that are used in audio have deadline
requirements for glitchless playback.  Those threads need to be
scheduled as RT tasks to ensure that they can meet the deadline even if
there is high system load.

Bug 37518404

Test:  play Bluetooth audio, check for RT with systrace
Change-Id: I4505fbce55e5a4fe18d00dbda23646a60e482efd