Merge "Update doc for mixer attributes related API." into udc-dev am: 25f63b50

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/21529711



Change-Id: I340b48c154b0ea444d703703f3c0154eddaecacf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>