Merge "AudioMixer: Enable 5.1 based for fallback downmix" am: 580a0d5508 am: 92447b3f4f

Original change: https://android-review.googlesource.com/c/platform/frameworks/av/+/2556150

Change-Id: I9800fb8f0289de0beec6f9d9d1cdb136a4280e2d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>