Fix kotlin nullable errors in SystemUI

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Change-Id: I8e3742e9a4a3d97d18e780b7aa88a75893d5b3ff
Test: builds
Merged-In: I39b7bc62d82e20b5ba8747be13a2aa7bf21a59a2