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
Change-Id: I26edfec35dca14abe90b08e3c74de0446eda95d2
Merged-In: I26edfec35dca14abe90b08e3c74de0446eda95d2
7 files changed