[various] Remove unnecessary material and cupertino imports

[ricardodalarme]

Remove material and cupertino imports from multiple files that don't need them

Part of: #flutter/flutter#191322

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Footnotes

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[gemini-code-assist]

Code Review

This pull request replaces imports of 'package:flutter/material.dart' with 'package:flutter/widgets.dart' across multiple packages, examples, and tests, reducing unnecessary dependencies on the Material library. There are no review comments, and I have no feedback to provide.