Migrate standard sample apps to UIScene lifecycle (#2884)

Fixes https://github.com/flutter/flutter/issues/176957

add_to_app/plugin/ios_using_plugin/IOSUsingPlugin/AppDelegate.swift

@@ -2,22 +2,36 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import UIKit
 import Flutter
 import FlutterPluginRegistrant
+import UIKit
 
-@UIApplicationMain
+@main
 class AppDelegate: UIResponder, UIApplicationDelegate {
-    var window: UIWindow?
-    
-    var flutterEngine : FlutterEngine?
-    
-    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
-        // Instantiate Flutter engine
-        self.flutterEngine = FlutterEngine(name: "io.flutter", project: nil)
-        self.flutterEngine?.run(withEntrypoint: nil)
-        GeneratedPluginRegistrant.register(with: self.flutterEngine!)
+  var flutterEngine: FlutterEngine?
 
-        return true
-    }
+  func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    // Instantiate Flutter engine
+    self.flutterEngine = FlutterEngine(name: "io.flutter", project: nil)
+    self.flutterEngine?.run(withEntrypoint: nil)
+    GeneratedPluginRegistrant.register(with: self.flutterEngine!)
+
+    return true
+  }
+
+  func application(
+    _ application: UIApplication,
+    configurationForConnecting connectingSceneSession: UISceneSession,
+    options: UIScene.ConnectionOptions
+  ) -> UISceneConfiguration {
+    let configuration = UISceneConfiguration(
+      name: "Default Configuration",
+      sessionRole: connectingSceneSession.role
+    )
+    configuration.delegateClass = FlutterSceneDelegate.self
+    return configuration
+  }
 }