Merge "AudioFlinger: Use optimized code for stereo downmix" am: c068aa5da5

Original change: https://android-review.googlesource.com/c/platform/frameworks/av/+/2552041

Change-Id: Ifa926a1c66b45853a5d8d16a1f3b989eefce03b5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>