
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: https://github.com/flutter/flutter/issues/137801 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style