Deterministic onLayout event ordering for iOS Paper (#40748)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/40748

The ordering of `onLayout` events is non-deterministic on iOS Paper, due to nodes being added to an `NSHashTable` before iteration, instead of an ordered collection.

We don't do any lookups on the collection, so I think this was chosen over `NSMutableArray` for the sake of `[NSHashTable weakObjectsHashTable]`, to avoid retain/release. Using a collection which does retain/release seems to cause a crash due to double release or similar, so those semantics seem intentional (though I'm not super familiar with the model here).

We can replicate the memory semantics with ordering by using `NSPointerArray` (which is unfortunately not parameterized). This change does that, so we get consistently top-down layout events (matching Fabric, and Android Paper as of D49627996). This lets us use multiple layout events to calculate right/bottom edge insets deterministically.

Changelog:
[iOS][Changed] -  Deterministic onLayout event ordering for iOS Paper

Reviewed By: luluwu2032

Differential Revision: D50093411

fbshipit-source-id: f6a9d5c973b97aede879baa8b952cc1be2447f28