Merge "AudioMixer: Enable 5.1 based for fallback downmix" into udc-dev am: ec503f6936 am: 66cc82b8dd

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

Change-Id: I20a4ed73703149cb88580406fd2cf64037f98517
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>