Allow duplicating thread to use native mixing audio format

Update OutputTrack to take variable formats.
Previously conversion to AUDIO_FORMAT_PCM_16_BIT was required.

Change-Id: I4a96a60ddd8d8dfe651405a0bcd4f98c89bc1ade
diff --git a/services/audioflinger/PlaybackTracks.h b/services/audioflinger/PlaybackTracks.h
index ee48276..902d5e4 100644
--- a/services/audioflinger/PlaybackTracks.h
+++ b/services/audioflinger/PlaybackTracks.h
@@ -255,7 +255,7 @@
 
     class Buffer : public AudioBufferProvider::Buffer {
     public:
-        int16_t *mBuffer;
+        void *mBuffer;
     };
 
                         OutputTrack(PlaybackThread *thread,
@@ -271,7 +271,7 @@
                                     AudioSystem::SYNC_EVENT_NONE,
                              int triggerSession = 0);
     virtual void        stop();
-            bool        write(int16_t* data, uint32_t frames);
+            bool        write(void* data, uint32_t frames);
             bool        bufferQueueEmpty() const { return mBufferQueue.size() == 0; }
             bool        isActive() const { return mActive; }
     const wp<ThreadBase>& thread() const { return mThread; }