Fix kotlin nullable errors in Launcher3

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:2608381792389b60ba37c08afcea09dca3c6ff9c)
Merged-In: I26edfec35dca14abe90b08e3c74de0446eda95d2
Change-Id: I26edfec35dca14abe90b08e3c74de0446eda95d2