[automerger skipped] Fixes @Nullable issues in System UI and WMShell. am: 0b00a5de9c -s ours

am skip reason: Merged-In I74cc61fd87819be7302590d788c3c230bff57544 with SHA-1 b84e4c0b3f is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/26585952

Change-Id: I5bf58ef6964d50fdde9a588f28d8af890320fd28
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>