Fix gapless playback

On some devices the actual framecount per buffer of an AudioTrack
will be different than what was requested, which prevented the
track from being reused. Now we create a new AudioTrack with the
requested parameters, and then compare it to the track we already
have. If they match, we throw away the new track and reuse the
existing one.
b/6644559

Change-Id: Id3e8c4460436f52e59b98ecaeb01c94f02877c1d