Fix crash in DownmixerBufferProvider

In ag/1768960, I forgot to actually set the buffers for the effect
interface. Without that, a 'process' method was attempting to use null
buffers.

Bug: 34228998
Test: android.media.cts.AudioNativeTest#testPlayStreamData
Change-Id: I904a9c08aa75bd5738b3ce981ee54511f37df7ec
diff --git a/media/libaudioprocessing/BufferProviders.cpp b/media/libaudioprocessing/BufferProviders.cpp
index 11ec367..8341a1e 100644
--- a/media/libaudioprocessing/BufferProviders.cpp
+++ b/media/libaudioprocessing/BufferProviders.cpp
@@ -200,6 +200,8 @@
          mEffectsFactory.clear();
          return;
      }
+     mDownmixInterface->setInBuffer(mInBuffer);
+     mDownmixInterface->setOutBuffer(mOutBuffer);
 
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