Add missing btif_a2dp_audio_interface_end_session() call during disconnect

Update the logic inside btif_a2dp_source_end_session_delayed() to make
sure that btif_a2dp_audio_interface_end_session() is called when device
is disconnected.

Also, update existing log messages to make it easier to identify similar
issues in the future.

Bug: 110176990
Test: Manual: Pair with two headsets, start/suspend/stop/switch active
      device
Change-Id: I248e5c3e2c4cf0ad820cef34ba859e9adc8dec8e