Fix kotlin nullable errors in SettingsLib

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:dbb8bfeac21c6be689c349cd59fe180889ccdf94)
Merged-In: I759f7022e500d64c4031c5b9244a00687c8631d7
Change-Id: I759f7022e500d64c4031c5b9244a00687c8631d7
5 files changed