Fix kotlin nullable errors in PackageInstallerSessions

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:9de59d6e94521e39bb5c30ec3e4d576e06a93df7)
Merged-In: Ia6e9d146eb5fd31b1c27664080c50398a147b1e4
Change-Id: Ia6e9d146eb5fd31b1c27664080c50398a147b1e4