AudioFlinger: refine mixer sleep time logic

When an AudioTrack is in underrun state, the AudioFlinger mixer will
sleep for a short period of time to give the app a chance to fill the
AudioTrack buffer. If the AudioTrack is still not ready during next mixing round,
the mixer will proceed with other tracks.

If an application keeps a steady underrun condition, the AudioFlinger mixer will
alternate between ready and not ready states. In the longer term this will cause the
audio HAL to underrun.
There is a mechanism to reduce the sleep period if the mixer is not ready several times in a
row but this mechanism is defeated by the alternating ready/not ready conditions.

The fix consists in only increasing sleep time if the mixer is ready for at least two
consecutive times.

Issue 5904527.

Change-Id: Id0139bca9be8c4e425ec6d428515c4d8f718e8c9