AudioTrack: fix unwanted underrun when restarting

When restarting an AudioTrack from stopped state, it is necessary
to force refresh of mRemainingFrames by processAudioBuffer() as
the last write before stop() could be partial. No doing so will lead
into unnecessary sleep before filling the non contiguous part
of the buffer returned by obtainBuffer() when processAudioBuffer()
is executed for the first time after start().

Change-Id: Id703f8dc092a6f07c905eee194054b4a978f979d