Fix kotlin nullable errors in EasterEgg

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:8db9adf852b00b403ae3e5b38a6f5d07d6c7a4b3)
Merged-In: Ia506c8d7b0a9009ada822108b9d85907ac18db2c

Change-Id: Ia506c8d7b0a9009ada822108b9d85907ac18db2c