[automerger skipped] Fixes @Nullable issues in System UI and WMShell. am: b84e4c0b3f -s ours am: 98d195e546 -s ours

am skip reason: Merged-In I74cc61fd87819be7302590d788c3c230bff57544 with SHA-1 507e0b7bac is already in history

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

Change-Id: I79d1781365ea5d1c91f9645f63de46d7fa8fe214
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>