Migrate FlutterViewController to ARC

[cbracken]

Migrates FlutterViewController from manual reference counting to ARC. Eliminates use of scoped_nsobject and scoped_nsprotocol, and migrates ivars to property syntax where possible.

No semantic changes, therefore no changes to tests.

Issue: https://github.com/flutter/flutter/issues/137801

Pre-launch Checklist

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [X] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [X] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [X] I listed at least one issue that this PR fixes in the description above.
• [X] 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.
• [X] We're about one patch away from vanquishing our ARCh enemy... MRC.
• [X] I updated/added relevant documentation (doc comments with ///).
• [X] I signed the CLA.
• [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard[bot]]

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!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[stuartmorgan-g]

test-exempt: code refactor with no semantic change

[jmagman]

I'm going to convert this to a draft, @cbracken can you mark it as ready when you'd like us to review?

[flutter-dashboard[bot]]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.