Explicitly stop streaming when switching streaming to another device

* Explicitly call btif_av_stream_stop() inside
  btif_a2dp_source_end_session_delayed() when ending the session.
  It should be called regardless whether we are using software
  or hardware based encoding.
* Removed extra btif_av_stream_stop() call inside
  BtifAvSource::SetActivePeer(), because it is not needed anymore.
  It is replaced by the call inside btif_a2dp_source_end_session_delayed()
* Added "peer_address" argument to btif_av_stream_stop()
  so it can be used to stop the stream to a specific peer.
* Fix an issue inside bta_av_chk_2nd_start() that was triggering
  extra start request to some headsets - the extra start request
  was blocking follow-up legitimate requests.
* Simplify the logic inside bta_av_do_start() so it is easier to
  identify issues when start streaming. The actual processing logic
  itself is not changed.
* Add extra log messages or update existing ones to help debug
  similar issues in the future.

Bug: 79909084
Test: Manual: streaming A2DP, connect a new device, change active device
Change-Id: Id583aa44aec7082dc5f94532e6b41a7ea33813a1