Cleanup DirectOutputThread::mActiveTrack

Rename activeTrack to mActiveTrack.
Release the reference earlier, at the end of threadLoop_mix().
This allows the field to be made private and to
  move the declaration from PlaybackThread to DirectOutputThread.

Change-Id: I02be7a254638f7d85e92aaf0002d20ca0092a5c3
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index e26466f..d91cd60 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -973,8 +973,6 @@
         mixer_state mPrevMixerStatus; // previous status returned by prepareTracks_l()
         // DIRECT only
         nsecs_t                         standbyDelay;
-        // activeTrack was local to the while !exitingPending loop
-        sp<Track>                       activeTrack;
         // DUPLICATING only
         uint32_t                        writeFrames;
     };
@@ -1046,6 +1044,9 @@
 
 private:
                     void        applyVolume();  // FIXME inline into threadLoop_mix()
+
+        // prepareTracks_l() tells threadLoop_mix() the name of the single active track
+        sp<Track>               mActiveTrack;
     };
 
     class DuplicatingThread : public MixerThread {