iOS: Migrate FlutterEngine to ARC (flutter/engine#55590)

Migrates `FlutterEnging` from manual reference counting to ARC. Migrates properties from `retain` to strong and `assign` to `weak` (where referencing an Obj-C object).

No semantic changes, therefore no changes to tests.

Issue: flutter#137801

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

engine/src/flutter/shell/platform/darwin/ios/BUILD.gn

@@ -70,7 +70,9 @@ source_set("flutter_framework_source_arc") {
     "framework/Source/FlutterDartVMServicePublisher.mm",
     "framework/Source/FlutterEmbedderKeyResponder.h",
     "framework/Source/FlutterEmbedderKeyResponder.mm",
+    "framework/Source/FlutterEngine.mm",
     "framework/Source/FlutterEngineGroup.mm",
+    "framework/Source/FlutterEngine_Internal.h",
     "framework/Source/FlutterHeadlessDartRunner.mm",
     "framework/Source/FlutterKeyPrimaryResponder.h",
     "framework/Source/FlutterKeySecondaryResponder.h",
@@ -186,8 +188,6 @@ 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/FlutterEngine.mm",
-    "framework/Source/FlutterEngine_Internal.h",
     "framework/Source/FlutterViewController.mm",
     "framework/Source/FlutterViewController_Internal.h",
     "platform_view_ios.h",