[image_picker] 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.

Introduces the same protocol abstraction now used in a number of our plugins, so that a stub can be injected without having to mock (which would be a barrier to Swift migration) or fake (which is fragile since it would have to be complete, and methods can be added over time) the entire Flutter plugin registrar.

Fixes flutter/flutter#174418

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Footnotes

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[gemini-code-assist]

Code Review

This pull request refactors the image_picker_ios plugin to improve UIScene compatibility by abstracting view controller access behind a FIPViewProvider protocol. This is a solid approach that also improves testability. The changes are mostly correct, but I've found a critical issue where many tests were not updated to use the new initializer, potentially masking issues and reducing test effectiveness. I've also included a couple of minor suggestions for improving code style and robustness.

[vashworth]

LGTM with a question: #10677 (comment)