Merge "Update gainmap documentation." into udc-dev am: 19573fe4 am: a46496b4

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/21701716



Change-Id: I82bb08c2e88d92e466a5b1d6b2f3e7d0e771deaa
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>