Fix a crash for a race condition during Bluetooth shutdown

This race condition is triggered when A2DP audio is streaming on shutdown:
"btif_a2dp_on_stopped() -> btif_media_task_aa_tx_flush_req()" is called
to stop the particular audio stream, and this happens right after
the "cleanup() -> btif_a2dp_stop_media_task()" processing during
the shutdown of the Bluetooth stack.

Bug: 22602117
Change-Id: I5de6a8f15b6a2771dde2e299a5b60554063696a2