Merge "Fixes more @Nullable issues in System UI and WMShell." into udc-qpr-dev am: ecb947c2

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24407292



Change-Id: I21658ce77a0d27d6c2ca5672d01e96235891a63d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>