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