iOS: Migrate FlutterEngineGroup to ARC

[cbracken]

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: flutter/flutter#137801

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
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[flutter-dashboard]

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[stuartmorgan]

test-exempt: code refactor with no semantic change

[cbracken]

FYI your comment says you changed this but it's still strong in the PR.

entrypointArgs should be strong; it's created by the FlutterEngineGroup and only ever used internally. I changed project to copy (for consistency with name, both of which are copied from the initialiser args.

[cbracken]

Rebased -- turns out FlutterEngineGroup is alphabetically right next to FlutterHeadlessEngine in the build file.

[stuartmorgan]

entrypointArgs should be strong; it's created by the FlutterEngineGroup and only ever used internally.

Internally to the engine maybe, but it's a public field of the class, and unless there's some very compelling reason, non-private collections should be copy. NSArray is CoW, so if it's never actually changed the copy is free anyway.

[cbracken]

Internally to the engine maybe, but it's a public field of the class.

Is that the case even if it's not exposed in a public header?

Ah sorry -- GitHub comments never cease to confuse me. I didn't look at the filename, and was mixing it up with my recollection of this field which is non-public:

@property(nonatomic, strong) NSMutableArray<NSValue*>* engines;

Alright, yes -- fixing the other one. Thanks for spotting.