iOS: Migrate FlutterAppDelegate to ARC

Migrates the FlutterAppDelegate.mm translation unit to be compiled with
the `-fobjc-arc` compiler flag.

Issue: flutter/flutter#137801

Author: cbracken

shell/platform/darwin/ios/BUILD.gn

@@ -59,6 +59,7 @@ source_set("flutter_framework_source_arc") {
   public_configs = [ "//flutter:config" ]
 
   sources = [
+    "framework/Source/FlutterAppDelegate.mm",
     "framework/Source/FlutterCallbackCache.mm",
     "framework/Source/FlutterCallbackCache_Internal.h",
     "framework/Source/FlutterChannelKeyResponder.h",
@@ -179,7 +180,6 @@ source_set("flutter_framework_source") {
     # iOS embedder is migrating to ARC.
     # New files are highly encouraged to be in ARC.
     # To add new files in ARC, add them to the `flutter_framework_source_arc` target.
-    "framework/Source/FlutterAppDelegate.mm",
     "framework/Source/FlutterEngine.mm",
     "framework/Source/FlutterEngineGroup.mm",
     "framework/Source/FlutterEngine_Internal.h",

shell/platform/darwin/ios/framework/Source/FlutterAppDelegate.mm

@@ -31,13 +31,6 @@ - (instancetype)init {
   return self;
 }
 
-- (void)dealloc {
-  [_lifeCycleDelegate release];
-  [_rootFlutterViewControllerGetter release];
-  [_window release];
-  [super dealloc];
-}
-
 - (BOOL)application:(UIApplication*)application
     willFinishLaunchingWithOptions:(NSDictionary*)launchOptions {
   return [_lifeCycleDelegate application:application willFinishLaunchingWithOptions:launchOptions];
@@ -286,7 +279,7 @@ - (id)forwardingTargetForSelector:(SEL)aSelector {
 - (void)logCapabilityConfigurationWarningIfNeeded:(SEL)selector {
   NSArray* backgroundModesArray =
       [[NSBundle mainBundle] objectForInfoDictionaryKey:kUIBackgroundMode];
-  NSSet* backgroundModesSet = [[[NSSet alloc] initWithArray:backgroundModesArray] autorelease];
+  NSSet* backgroundModesSet = [[NSSet alloc] initWithArray:backgroundModesArray];
   if (selector == @selector(application:didReceiveRemoteNotification:fetchCompletionHandler:)) {
     if (![backgroundModesSet containsObject:kRemoteNotificationCapabitiliy]) {
       NSLog(