This migrates FlutterEngineGroup to automatic reference counting. It also migrates `enginesCreatedCount` from an ivar to property syntax. I'll look at migrating engine from `NSMutableArray<NSValue*>` to a weak pointer array in a followup patch since it's orthogonal to ARC migration. Issue: https://github.com/flutter/flutter/issues/137801 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style