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
Merged-In: I88aec6fbc814be98f9ef048bbc5af889d0797970
Change-Id: I158db4f57d46606d7152a438d68e2b9f4c0014d9