Fix regression for SoundPool playback

Commit 19ddf0eb introduced a problem with applications (like SoundPool)
relying on an underrun condition to detect end of playback instead of
stopping the track when all data is written.
AudioFlinger would keep waiting for new data in case of partial buffer
filling and never reach the underrun condition.

Added a mechanism to wait no more than once if not enough frames are present
in the track buffer.

Issue 5585490.

Change-Id: I131e605ff6070831a01ddf734e68459e3bf2354b