Update FlutterSurveyService.java

[jayoung-lee]

Update the URL for the survey metadata.

The flutter.dev website was updated and the old link doesn't work anymore.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• 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.
• All existing and new tests are passing.

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[kenzieschmoll]

@stevemessick actually already has a PR out that does this: #5863. Closing this in favor of that one - thanks!