Fix issue 1795088       Improve audio routing code

Initial commit for review.
Integrated comments after patch set 1 review.
Fixed lockup in AudioFlinger::ThreadBase::exit()
Fixed lockup when playing tone with AudioPlocyService startTone()
diff --git a/libs/audioflinger/AudioMixer.h b/libs/audioflinger/AudioMixer.h
index 72ca28a..15766cd 100644
--- a/libs/audioflinger/AudioMixer.h
+++ b/libs/audioflinger/AudioMixer.h
@@ -85,6 +85,8 @@
 
     uint32_t    trackNames() const { return mTrackNames; }
 
+    static void ditherAndClamp(int32_t* out, int32_t const *sums, size_t c);
+
 private:
 
     enum {
@@ -176,7 +178,6 @@
     static void volumeRampStereo(track_t* t, int32_t* out, size_t frameCount, int32_t* temp);
     static void track__16BitsStereo(track_t* t, int32_t* out, size_t numFrames, int32_t* temp);
     static void track__16BitsMono(track_t* t, int32_t* out, size_t numFrames, int32_t* temp);
-    static void ditherAndClamp(int32_t* out, int32_t const *sums, size_t c);
 
     static void process__validate(state_t* state, void* output);
     static void process__nop(state_t* state, void* output);