[iOS] Eliminate ARC __bridge casts where possible

[cbracken]

During ARC migration in #137801 we left behind some unfortunate __bridge casts. In the interest of safety and readability, we should eliminate these where possible.

[jmagman]

Following the link from https://github.com/flutter/engine/blob/033fa28a58bb775a519dc5fde148d5b4dbee3b20/shell/platform/darwin/ios/framework/Source/FlutterEngineGroup.mm#L86-L87
If self.engines is swapped to a [NSPointerArray weakObjectsPointerArray] maybe the kFlutterEngineWillDealloc notification can be totally deleted?

[cbracken]

Yep - I was looking at doing just that; the one issue with NSPointerArray is that it doesn't have a remove by value message, just by index. Probably still worth it though.

[github-actions]

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