Downmix Effect: Fix processed samples

For downmix, the output buffer size can be diff with input

Bug: 318926783
Bug: 317946442
Test: atest --test-mapping hardware/interfaces/audio/aidl/vts:presubmit
Change-Id: I8aadcd64ddb460b8b1d9d80086c4edbc73cf11cf
Merged-In: I8aadcd64ddb460b8b1d9d80086c4edbc73cf11cf
1 file changed