Merge "Downmix: Add AIDL placeholder implementation and vts test" am: 1ad0ff0f79 am: a1b3286a15

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/2309207

Change-Id: Icb7f19f370b5437d91a3b013e7b55f094fa01b91
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>