…#180041)

This PR listens to changes to the value of the `(prefers-reduced-motion:
reduce)` media query to update the `reduceMotion` and
`disableAnimations` properties of the `AccessibilityFeatures`
configuration object.

This is achieved by introducing a new `MediaQueryManager` object to the
`EnginePlatformDispatcher` that handles handles all the engine
configuration values that are driven by media queries: "reduced motion",
"high contrast" and "dark mode".

Unifying the code paths of all these values, allows this PR to slightly
increase testing coverage of those values, by injecting fake browser
events in the `platform_dispatcher_test.dart` file.

### Issues

* Fixes flutter#167566

### Testing

* Added browser tests for new (and existing) media-queries
* Deployed test app to: https://dit-tests.web.app (may get offline!)

## Pre-launch Checklist

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

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)

…lutter/flutter#180041)