Fixes more  @Nullable issues in System UI and WMShell. am: 63a020ccfb

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24588595

Change-Id: I3cc1f71060dcff2df2f167dfbf2c8c72f9faa4ef
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>