Free the A2DP Source media alarm on the A2DP Source worker thread

Move the alarm_free() for the A2DP Source media alarm from
btif_a2dp_source_shutdown() to btif_a2dp_source_shutdown_delayed().
The latter is executed on the btif_a2dp_source_thread.
This avoids a race condition with btif_a2dp_source_audio_tx_stop_event()
that also might attempt to free the same alarm.

Test: Manual - unpair A2DP device while A2DP streaming.
Bug: 73741079
Change-Id: Ia871fcbeaae7c351878228e7605e65307a3d1583