Fix more 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:4aef83aa32df00c3d3e7c4cc239a08299c3b7d7d)
Merged-In: I158db4f57d46606d7152a438d68e2b9f4c0014d9
Change-Id: I158db4f57d46606d7152a438d68e2b9f4c0014d9