Add Start/End Session for A2DP offload audio interface

* Added explicit btif_a2dp_audio_interface_start_session() and
  btif_a2dp_audio_interface_end_session() and call them as appropriate.
* Removed unnecessary btif_a2dp_audio_interface_deinit() calls.

Bug: 74952724
Test: Manual: audio streaming without A2DP offload
Change-Id: I74b8c340ee258b09b9e1acbcb1cc75906ddc03fc