This refactors how high contrast is implemented on Windows:
1. Added a test to verify accessibility features are updated when a view is created. This prevents staleness issues as the Windows embedder isn't notified of accessibility changes while in headless mode.
1. Moved high contrast mode detection to `WindowsProcTable` from `FlutterWindow` to remove engine to view to window plumbing.
1. `FlutterWindow` and `FlutterWindowsEngine` now share their `WindowsProcTable` (which is used for mocking and polyfilling win32 APIs) to reduce redundant dynamic loading.
This pull request contains no functional changes.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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