[iOS] Inject DisplayLinkManager into FlutterKeyboardInsetManager

[cbracken]

This is a follow-up to the DisplayLinkManager singleton migration (#189492). FlutterKeyboardInsetManager was reading DisplayLinkManager.shared directly in setUpKeyboardAnimationVsyncClient when passing its VSyncClient's isVariableRefreshRateEnabled and maxRefreshRate, which is effectively using a global.

This wires a DisplayLinkManager through the initializer instead and stores it in an instance var, so that read goes through the injected instance.

I didn't need to touch FakeFlutterKeyboardInsetManager. It's an independent NSObject that conforms to
FlutterKeyboardInsetManagerProtocol and not a subclass, so it never depended on this initializer.

This is a refactoring so introduces no semantic changes. I've updated tests to use the new pattern but there's no new functionality to test.

Issue: #175879
Issue: #181684

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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Code Review

This pull request refactors KeyboardInsetManager to inject DisplayLinkManager via its initializer, replacing direct references to the global DisplayLinkManager.shared singleton. The FlutterViewController and its unit tests are updated to pass the shared instance. Feedback suggests taking advantage of this dependency injection in tests by mocking DisplayLinkManager instead of using the global singleton, which would make the tests hermetic and independent of the host device's display capabilities.