Merge "Adds explanation of MessagingStyle fields" into tm-dev am: d7356c9e am: 1a1e05d8 am: 3d05a968

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/18008166



Change-Id: Id9d12c6144e22fdb6d7cade4fb4b40dac11c0059
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>