AudioFlinger: Properly account for underruns

When the mixer status was MIXER_ENABLED twice in a row
zeroes were forced to the audio sink.

Test: aaudio test write_sine_callback -pl -n1 -r44100 -s20
      and then use rapid volume changes.
Bug: 131767115
Change-Id: I24b41084138210c63e8c551ae4a5f3adddb9e607