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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2d4a9f40c357e1979895fac005934c1185461798)
Merged-In: If13ac9b81cfe65730d5ce270e9409bc97cef75ec

Change-Id: If13ac9b81cfe65730d5ce270e9409bc97cef75ec