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.

This relands I26edfec35dca14abe90b08e3c74de0446eda95d2 with a fix in
SplitSelectDataHolder.kt to call createPackageContext when user is null
instead of asserting that it is not null.

Bug: 294110802
Test: builds
Test: WMShellFlickerServiceTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4cc251e6c1bd5e06ef24a92e08aae043cf3f2587)
Merged-In: I4525d0fa83a1db9cc5cff90f340fc3f863537c01
Change-Id: I4525d0fa83a1db9cc5cff90f340fc3f863537c01