Merge "Fixes more @Nullable issues in System UI and WMShell." into...

Merge "Fixes more  @Nullable issues in System UI and WMShell." into udc-qpr-dev am: ecb947c2 am: a3033abc

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



Change-Id: I83c49d222301938c1a03cb6c200405e82d0092c7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>