Merge "Update clock plugin documention in SystemUI" into udc-dev am: f1d08d5d

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/21957272



Change-Id: I9f851a9d845f3fee4c7db9f107505dd8631b882a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>