Change the heuristics for waiting for audiotracks to finish.

This is made necessary by a bug when the utterance is smaller
than the audio buffer size. In that case, we call stop() to
flush the audio to the mixer, but that causes the playstate to
be set to stopped though some audio is still being mixed. This
breaks our waiting loop.

We now wait a fixed amount of time for such short utterances
and do not observe the playback head position.

bug:5220048
Change-Id: Ic81dec751c1faca0b14164caeda6305c8f9815fe