AudioFlinger: fix tracks ready for mixing logic.

Commit fec279f5 broke the logic allowing to wait for an application
to provide frames for mixing in the case of several active tracks.

This was causing audio gaps when playing music and superposing a
sound Fx (keyboard clicks...).

Issue 6185007.

Change-Id: Id0fad150d0b615646d6b1387c0de8ca944d228f6
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index 23fc74d..2a85115 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -1052,8 +1052,8 @@
         uint32_t                        sleepTime;
 
         // mixer status returned by prepareTracks_l()
-        mixer_state                     mMixerStatus;       // current cycle
-        mixer_state                     mPrevMixerStatus;   // previous cycle
+        mixer_state                     mMixerStatus; // current cycle
+                                                      // previous cycle when in prepareTracks_l()
 
         // FIXME move these declarations into the specific sub-class that needs them
         // MIXER only