[pigeon] fixes Java generator uses wrong style for enum values

[cofirazak]

Pigeon Java code generator uses wrong style (camelCase from Dart) for enum values. This PR fixes it to UPPER_CASE naming convention for enum values.

Fixes #105801

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 [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
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• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• [x] I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• [ ] 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] All existing and new tests are passing.

[cofirazak]

@stuartmorgan IDKW, but there was no reviewers added automatically to this PR. And i don't see how can i add someone manually.

[stuartmorgan]

There's no auto-assignment set up for this repository. We do a weekly triage and route to reviewers that way.

[flutter-dashboard[bot]]

This pull request is not mergeable in its current state, likely because of a merge conflict. Pre-submit CI jobs were not triggered. Pushing a new commit to this branch that resolves the issue will result in pre-submit jobs being scheduled.