[video_player] Update for UIScene compatibility

[stuartmorgan-g]

Replaces the code that used the key window's root view controller with a call to the new registrar viewController method to get the actual Flutter content's view controller.

Since code behind the view provider abstraction is by definition not unit testable (since that's the interface used to DI a replacement in tests), this changes the API surface so that it's a direct passthrough to the registrar method. This does add some additional ifdefing, but minor compile-time complexity is better than having additional logic behind the test injection abstraction.

Fixes flutter/flutter#174416

Pre-Review Checklist

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Footnotes

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[gemini-code-assist]

Code Review

This pull request updates the video_player_avfoundation plugin for UIScene compatibility by replacing the deprecated keyWindow API with registrar.viewController. The changes are correctly implemented in the native code and the tests are updated accordingly, improving compatibility with modern iOS practices. I have one suggestion to align an initializer in the test code with Objective-C best practices.