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Notable changes:
- `FlutterTextInputPlugin` is now owned by `FlutterEngine`.
- `FlutterTextInputPlugin` is only associated with a
`FlutterViewController` while having active IME connection.
- Changed private state of `TextInputPlugin` from properties to ivars.
There is no need for these to be properties, and the state was accessed
both as ivars and properties which seemed inconsistent.
- Updated test to reflect the changes

I've also tested everything including voice-over to make sure
accessibility didn't regress.

## 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], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

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[Flutter Style Guide]:
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[Features we expect every widget to implement]:
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[Discord]:
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