Migrate testing_app to UIScene lifecycle (#2836)

## Summary
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- Migrate `testing_app` iOS project to the UIScene lifecycle as per the
[migration
guide](https://docs.flutter.dev/release/breaking-changes/uiscenedelegate)
- Update `AppDelegate.swift` to use `@main`,
`FlutterImplicitEngineDelegate`, and move plugin registration to
`didInitializeImplicitFlutterEngine`
- Add `UIApplicationSceneManifest` to `Info.plist`

Part of flutter/flutter#176957

## Test plan
- [x] Ran `testing_app` on iOS simulator — app launches and works
correctly

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

testing_app/ios/Runner/AppDelegate.swift

@@ -1,13 +1,16 @@
-import UIKit
 import Flutter
+import UIKit
 
-@UIApplicationMain
-@objc class AppDelegate: FlutterAppDelegate {
+@main
+@objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
   override func application(
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
-    GeneratedPluginRegistrant.register(with: self)
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }
+
+  func didInitializeImplicitFlutterEngine(_ engineBridge: FlutterImplicitEngineBridge) {
+    GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
+  }
 }