Fix kotlin nullable errors in ThemePicker

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

Bug: 294110802
Test: builds
Change-Id: If13ac9b81cfe65730d5ce270e9409bc97cef75ec
Merged-In: If13ac9b81cfe65730d5ce270e9409bc97cef75ec