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

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

Change-Id: I189990579ca57e4faec0c32a47b74f60e8cd3f75
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>