Split btif_media_task into Source, Sink and Control

* btif/include/btif_a2dp.h and btif/src/btif_a2dp.cc implement
  the entry points for the BTIF A2DP module.

* btif/include/btif_a2dp_source.h and btif/src/btif_a2dp_source.cc implement
  the BTIF A2DP Source component.

* btif/include/btif_a2dp_sink.h and btif/src/btif_a2dp_sink.cc implement
  the BTIF A2DP Sink component.

* btif/include/btif_a2dp_control.h and btif/src/btif_a2dp_control.cc
  implement the A2DP control mechanism for the audio channel from the
  Media Framework.

Also:
* Removed BTA_AV_SBC_HDR_SIZE and used A2D_SBC_MPL_HDR_LEN instead.
* Removed BTIF_AV_SINK_FOCUS_REQ_EVT, because it is not used.
* Removed many of the "#if (BTA_AV_SINK_INCLUDED == TRUE)" guards.
* Removed "#ifdef USE_AUDIO_TRACK" guard, and always compile the
  corresponding code.
* Removed tBTIF_AV_MEDIA_FEEDINGS_PCM_STATE and moved its state
  to tBTIF_AV_MEDIA_FEEDINGS_STATE .

Bug: 30958229
Test: TestTracker/65192
Change-Id: I20bc52a1d7a7f03c92628a1562f14b7df3ebb445