aaudio: stop buzzing when MMAP stream disconnected

When a stream is disconnected, clear the MMAP memory
buffer to prevent the hardware from looping on stale data
and making a buzzing sound.

Bug: 63342351
Test: play GStomper with AAudio through headphones and then unplug them
Change-Id: I6372b0a031dea242e3830cc2d226a4c043d4467e