restructured the add to app samples (#698)

add_to_app/plugin/ios_using_plugin/IOSUsingPlugin/AppDelegate.swift

@@ -0,0 +1,23 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// 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
+
+@UIApplicationMain
+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!)
+
+        return true
+    }
+}