Fix issue 1970108: crash in AudioFlinger::isMusicActive()

Protected calls to MixerThread::isMusicActive() by AudioFlinger::mLock
diff --git a/libs/audioflinger/AudioFlinger.h b/libs/audioflinger/AudioFlinger.h
index 8e47b29..634934e 100644
--- a/libs/audioflinger/AudioFlinger.h
+++ b/libs/audioflinger/AudioFlinger.h
@@ -463,7 +463,7 @@
         virtual     float       streamVolume(int stream) const;
         virtual     bool        streamMute(int stream) const;
 
-                    bool        isMusicActive() const;
+                    bool        isMusicActive_l() const;
         
                     
                     sp<Track>   createTrack_l(