btif_a2dp_source: Run a2dp::set_audio_low_latency_mode_allowed on bt_a2dp_source_worker_thread

a2dp::cleanup is run from bt_a2dp_source_worker_thread and executing
a2dp::set_audio_low_latency_mode_allowed on a different thread
will open a potential data race.

Bug: 329537023
Test: m com.android.btservices
Flag: EXEMPT, minor bugfix
Change-Id: I164532c642b26a8cd73d14d6acc4a9e102298896