Fixes @Nullable issues in System UI and WMShell. am: 549f19a8a5

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

Change-Id: I6ff55d04528bad0289202e495f4c95e8718a63ea
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>