Use FlutterAppDelegate in iOS create template (#7845)

On iOS, generate a FlutterAppDelegate subclass for the application
delegate. This avoids touchesBegan:withEvent boilerplate to handle
status bar taps.

packages/flutter_tools/templates/create/ios.tmpl/Runner/AppDelegate.h

 #import <UIKit/UIKit.h>
+#import <Flutter/Flutter.h>
 
-@interface AppDelegate : UIResponder <UIApplicationDelegate>
-
-@property (strong, nonatomic) UIWindow *window;
+@interface AppDelegate : FlutterAppDelegate
 
 @end

packages/flutter_tools/templates/create/ios.tmpl/Runner/AppDelegate.m

-#import <Flutter/Flutter.h>
 #include "AppDelegate.h"
 
 @implementation AppDelegate
@@ -30,16 +29,4 @@
     // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
 }
 
-- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
-  [super touchesBegan:touches withEvent:event];
-
-  // Support scroll to top on status bar tap. By default, pass status bar taps
-  // to the application's root view controller for handling in Flutter.
-  UIViewController *viewController =
-  [UIApplication sharedApplication].keyWindow.rootViewController;
-  if ([viewController isKindOfClass:[FlutterViewController class]]) {
-    [(FlutterViewController*)viewController handleStatusBarTouches:event];
-  }
-}
-
 @end