implement flow control to avoid dropping packets in btif_media_task.

implment flow conrol to save CPU usage and avoid dropping
packet in btif_media_task when overflow and cpu preemption happen.
when the packet count in the queue is above high watermark, we will
stopping adding packets, when the packet count in the queue is below
low watermark, we will start adding packets again.

bug:8252054
Change-Id: I3b7795dffbe5f2ae455e239a0234533f6cbb4403