iOS: Migrate FlutterEngineGroup to ARC

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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.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I secretly hope people will start referring to me as the dARC knight.
• 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.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

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