Isolate references to outputTracks/mOutputTracks

Move all references to DuplicatingThread::outputTracks and
DuplicatingThread::mOutputTracks from the common threadLoop() into
virtual methods.  This allows them to be moved from PlaybackThread to
DuplicatingThread, and to be marked private.

Also use vector assignment to copy mOutputTracks to outputTracks.

Change-Id: Ieb1cf1ad36b8a65143e61e6c92a65fb43427e5e2