Add Gamepad support for the activation action

[creativecreatorormaybenot]

Description

In native apps, the A button on gamepads will trigger the activation action, e.g. when selecting semantic elements on the screen.

Related Issues

• Resolves Accessibility: Selected buttons cannot be triggered on Game Controller #47697.

Tests

I added the following tests:

• Confirming that the default shortcut mapping properly works.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.

[gspencergoog]

Thanks for the PR!

[gspencergoog]

Your test fails on web because on web, enter doesn't trigger activation (by design). You'll have to check to see if you're running on web in the test using kIsWeb, and only test for enter if it's false.

[creativecreatorormaybenot]

@gspencergoog Yes, of course. I saw it, but forgot to consider it. Thanks for pointing it out and thanks for the review 👍

[gspencergoog]