[automerger skipped] Fixes more @Nullable issues in System UI and WMShell. am: d4e65759 -s ours

am skip reason: Merged-In Ibd2b0163b47571da5f276f09ce5dcfd875f09a1c with SHA-1 10c09e60 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24589269



Change-Id: I3bd297e43e7234828b75da33b8eb611fe3451047
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>