audio hal: Fix multichannel playback

Correctly operate on input and output buffers in DownmixerBufferProvider

Note that playback is a bit choppy, need to investigate further.

Change-Id: I350175dcc9cc7142a1935585a8bd5e9abb1b8eb6
Test: play back a 5.1 file
diff --git a/media/libaudiohal/EffectBufferHalLocal.h b/media/libaudiohal/EffectBufferHalLocal.h
index df7bd43..202d878 100644
--- a/media/libaudiohal/EffectBufferHalLocal.h
+++ b/media/libaudiohal/EffectBufferHalLocal.h
@@ -35,6 +35,8 @@
 
     virtual void update();
     virtual void commit();
+    virtual void update(size_t size);
+    virtual void commit(size_t size);
 
   private:
     friend class EffectBufferHalInterface;