Merge "Fixes @Nullable issues in System UI and WMShell." into udc-qpr-dev am: f2fc5d94

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24363286



Change-Id: I77596f28e62c7ce59a2336a6e214956a6a1060cf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>