Merge "[sf] Add readme for surfaceflinger frontend" into udc-dev am: 0f0d7b03 am: 41fb2c90

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/23272393



Change-Id: Ie15efdb8c827128a1a062a3a12a1cdf47e2d6074
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>