Fix kotlin nullable errors in SilkFX tests

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: I8113aa76a60904bc70211f95b1e4c05d11e4ae85
Merged-In: I8113aa76a60904bc70211f95b1e4c05d11e4ae85