Added Start/End Session steps to A2DP session setup

Added Start/End Session steps to A2DP session setup

Start/End session is called when setting/changing the Active (remote)
device.

Also:
 * For A2DP Source, btif_a2dp_source_setup_codec() is called only for
   Start session. All other calls to btif_a2dp_source_setup_codec are
   removed.
 * Updated the btif_a2dp_source_setup_codec() implementation to
   call btif_a2dp_source_audio_tx_flush_req() and flush the incoming audio
   data. This removes 2 seconds hold-up delay when switching the active
   device and streaming audio.
 * Removed unnecessary lock inside btif/src/btif_a2dp_sink.cc

Bug: 74952724
Test: Manual - Connect two headsets, switch active device,
      connect/disconnect.
Change-Id: I43702e1ddc108628de93161905465647471f554c