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.cpp b/media/libaudiohal/EffectBufferHalLocal.cpp
index 20b1339..7e6ee85 100644
--- a/media/libaudiohal/EffectBufferHalLocal.cpp
+++ b/media/libaudiohal/EffectBufferHalLocal.cpp
@@ -75,4 +75,10 @@
 void EffectBufferHalLocal::commit() {
 }
 
+void EffectBufferHalLocal::update(size_t size) {
+}
+
+void EffectBufferHalLocal::commit(size_t size) {
+}
+
 } // namespace android