[automerger skipped] Fixes @Nullable issues in System UI and WMShell. am: 549f19a8a5 am: b998445fec -s ours am: 37baf5b0b7 -s ours

am skip reason: Merged-In I74cc61fd87819be7302590d788c3c230bff57544 with SHA-1 162280b4f9 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24592300

Change-Id: I311dc1253e49e6ad374d13ddf30fd09d25228f69
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>