Merge "AudioMixer: Enable 5.1 based for fallback downmix" into udc-dev am: ec503f6936 am: e7fe981e03

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/av/+/22688852

Change-Id: Ieffa65a78182c8595b8af5cbb0214bf55f1c8d06
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>