[a11y] fix selected tristate flags mismatch

[hannah-hyj]

fix #184058
The issue and solution was both pointed out in #184058

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• 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. 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.

[gemini-code-assist]

Code Review

This pull request updates the accessibility bridge to handle the is_selected flag as a FlutterTristate and adds a unit test to verify the logic. A review comment points out that the tests use C++20 designated initializers, which should be replaced with member-wise assignments to comply with the project's C++17 standard and the Google C++ Style Guide.

[fluttergithubbot]

An existing Git SHA, f14a69e97cbbae69fa185a8bfa0f732a5ec48060, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[chunhtai]

LGTM