aaudio: fix getXRunCount

Mixer was not advancing FIFO when underflowed so the client could not
detect the xrun.
Also underruns could cause the stream to drift in time relative to other
streams.

Bug: 64033855
Test: trigger underruns by sleeping in a callback
Change-Id: Ia038f5048ffbff3b82e9d86e6807483dfe6879d0