[automerger skipped] Fixes @Nullable issues in System UI and WMShell. am: b84e4c0b -s ours

am skip reason: Merged-In I74cc61fd87819be7302590d788c3c230bff57544 with SHA-1 507e0b7b is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24692799



Change-Id: I59f85c909b055aa90800915ee4466770a01d1be4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>