Fixes @Nullable issues in System UI and WMShell.

Test: followed these steps:
1. Patched in https://r.android.com/2688146
2. m sysuig
3. Fix compilation issues
4. Repeat 2 and 3 until done

Fix: 294098415
(cherry picked from commit 162280b4)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:549f19a8a5653972af79f39178091d985e757407)
Merged-In: I74cc61fd87819be7302590d788c3c230bff57544
Change-Id: I74cc61fd87819be7302590d788c3c230bff57544