Prepare for threadLoop merge - active tracks

Continued work on making the copies of threadLoop more similar:
 - Remove alias for mActiveTracks in MixerThread and DuplicatingThread.
 - Pull in declaration of activeTrack in DirectOutputThread.
 - Remove redundant parameter of prepareTracks_l().
 - Comment prepareTracks_l().

Change-Id: If1087c1902b454acec01ddfdd9f055f0ca7abf04