Use case

I came across these functions in FlutterAppDelegate.mm:

- (BOOL)application:(UIApplication*)application shouldSaveApplicationState:(NSCoder*)coder {
  [coder encodeInt64:self.lastAppModificationTime forKey:kRestorationStateAppModificationKey];
  return YES;
}

These APIs are also deprecated, and we should check if they are still working after adopting SceneDelegate.

Unlike other APIs, the new state restoration API seems to be a non-trivial change: https://developer.apple.com/documentation/uikit/restoring-your-app-s-state

CC @gaaclarke @vashworth

Proposal

1. Double check if the existing state restoration still works after adopting scene delegate
2. Migrate to SceneDelegate APIs