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/include/media/audiohal/EffectBufferHalInterface.h b/include/media/audiohal/EffectBufferHalInterface.h
index 102ec56..6fa7940 100644
--- a/include/media/audiohal/EffectBufferHalInterface.h
+++ b/include/media/audiohal/EffectBufferHalInterface.h
@@ -42,6 +42,8 @@
 
     virtual void update() = 0;  // copies data from the external buffer, noop for allocated buffers
     virtual void commit() = 0;  // copies data to the external buffer, noop for allocated buffers
+    virtual void update(size_t size) = 0;  // copies partial data from external buffer
+    virtual void commit(size_t size) = 0;  // copies partial data to external buffer
 
     static status_t allocate(size_t size, sp<EffectBufferHalInterface>* buffer);
     static status_t mirror(void* external, size_t size, sp<EffectBufferHalInterface>* buffer);