DownmixBufferProvider: Assign buffer frame count

Not specifying buffer count leads to opening of DownmixEffect
with input/output FMQs of size zero (in AIDL version).

Bug: 302132812
Test: atest CtsMediaAudioTestCases --test-filter=".*AudioTrackTest.*"
Change-Id: Ic450bd918286237eff9ee43c84e7c7f54bcf9cdf
diff --git a/media/libaudioprocessing/BufferProviders.cpp b/media/libaudioprocessing/BufferProviders.cpp
index 9f19f7b..fbc7f90 100644
--- a/media/libaudioprocessing/BufferProviders.cpp
+++ b/media/libaudioprocessing/BufferProviders.cpp
@@ -185,6 +185,8 @@
      mDownmixConfig.inputCfg.mask = EFFECT_CONFIG_SMP_RATE | EFFECT_CONFIG_CHANNELS |
              EFFECT_CONFIG_FORMAT | EFFECT_CONFIG_ACC_MODE;
      mDownmixConfig.outputCfg.mask = mDownmixConfig.inputCfg.mask;
+     mDownmixConfig.inputCfg.buffer.frameCount = bufferFrameCount;
+     mDownmixConfig.outputCfg.buffer.frameCount = bufferFrameCount;
 
      mInFrameSize =
              audio_bytes_per_sample(format) * audio_channel_count_from_out_mask(inputChannelMask);