TimedAudio: Fix a cause of audio popping.

This is a manual merge from ics-aah

> TimedAudio: Fix a cause of audio popping.
>
> Fix an issue with buffer lifecycle management which could cause audio
> pops on timed outputs.  There were two issues at work here.
>
> 1) During trim operations for the queued timed audio data, buffers
>    were being trimmed based on their starting PTS instead of when the
>    chunk of audio data actually ended.  This means that if you have a
>    very large chunk of audio data (larger than the mixer lead time),
>    then a buffer at the head of the queue could be eligible to be
>    trimmed before its data had been completely mixed into the output
>    stream, even though the output stream was fully buffered and in no
>    danger of underflow.
> 2) The implementation of getNextBuffer and releaseBuffer for timed
>    audio tracks was not keeping anything like a reference to the data
>    that it handed out to the mixer.  The original architecture here
>    seemed to be expecting a ring buffer design, but timed audio tracks
>    use a packet based design.  Pieces of packets are handed out to the
>    mixer which then frequently will hold onto that chunk of data
>    across two mix operations, using the first part of the chunk to
>    finish a mix buffer and then using the end of the chunk for the
>    start of the next mix buffer.  If the buffer that the mixer is
>    holding a piece of got trimmed before the start of the next mix
>    operation, it would return to its heap and could be filled with who
>    knows what by the time it actually got mixed.  On debug builds,
>    they seem to get zero'ed out as they go back to the heap causing
>    obvious pops in presentation.
>
> This change addresses both issues.  Trim operations are now based on
> ending presentation time for a chunk of audio, not the start.  Also,
> when the head of the queue is in flight to the mixer, it can no longer
> be trimmed immediately, merely flagged for trim by the mixer when the
> mixer finally does call releaseBuffer.
>
> Signed-off-by: John Grossman <johngro@google.com>
> Change-Id: Ia1ba08cb9dea35a698723ab2d9bcbf804f1682fe

Change-Id: I2c5e2f0375c410f0de075886aac56ff6317b144c
Signed-off-by: John Grossman <johngro@google.com>