audioflinger: reduce sleep time to avoid underrun

Progressively reduce the sleep time applied in MixerThread::threadLoop()
in case of consecutive application underruns to avoid starving the audio HAL.
As the default sleep time is longer than the duration of an audio buffer
we ended up writing less data than needed by the audio HAL if
the condition persisted.

Issue 5553055.

Change-Id: I2b23ee79c032efa945025db228beaecd1e07a2e5