AudioTrack: Fix callback timing if less than one AF mixer period.

Callback may wait nearly the entire period when double buffered
which can lead to random glitches.  Limit this to half the mixer period.

Test: aaudio test write_sine_callback -pl -n1 -r44100 -s20
Bug: 129545307
Bug: 131601263
Change-Id: I6b7478728fe7ead570251662ee48a8f8adad0d5d