[audio] Offload thread signal to prevent deadlock

On an appops changed callback, we need to signal the audio threadloop
for tracks which don't sync write (MMAP and offload). Since this signal
grabs the thread mutex, it isn't legal to do in the callback due to lock
ordering.

Instead, enqueue a task onto an executor per offload/mmap track which
appropriately signals the thread without holding any external locks.

Test: Manual playback muting test for MMAP
Flag: EXEMPT bugfix
Fixes: 391668615
Change-Id: Icfd39957d0667f69bd42eba95dbbaa54136fff4c