Fix crash on iOS when UISceneDelegate lifecycle is adopted

[m-sugawara]

Fix #145

Updated presentationAnchor(for:) to retrieve the window in the following priority order:

1. FlutterPluginRegistrar.viewController - Use the view controller's window provided by Flutter (most reliable)
2. UIScene-based retrieval (iOS 13+) - Iterate through connected scenes to find the active foreground scene's key window
3. Legacy fallback - Fall back to AppDelegate.window for apps not using UIScene lifecycle