Fixes more  @Nullable issues in System UI and WMShell.

Test: builds
Fix: 294098415
Change-Id: Ibd2b0163b47571da5f276f09ce5dcfd875f09a1c
Merged-In: Ibd2b0163b47571da5f276f09ce5dcfd875f09a1c
(cherry picked from commit 10c09e60e58df9ddb8e2ebce7537cb6f5142456c)
1 file changed