[pigeon] Consolidate platform test harnesses part 1 - harnesses

[stuartmorgan-g]

Part 1 of #2803

Adds two plugins—one for most languages, one for alternate languages (Java and Obj-C)—to serve as native test hosts, to consolidate everything into a multi-platform test harness that uses exactly the same structure as all of the native unit tests in flutter/plugins.

This PR is a no-op in terms of the actual test script; it adds the scaffolding with nothing using it yet. Each test will be moved in a separate follow-up PR to isolate the CI issues in #2803. This portion corresponds to:

• Adding the two new test plugin projects, using flutter create.
• Cleaning up those templates (autoformat, license boilerplate, removing unneeded files, adding README info, etc.).
• Rolling back the Gradle and AGP versions to avoid the Java 11 problem discussed in [pigeon] Consolidate platform test harnesses #2803.

This does preemptively remove a Cirrus CI workaround for dart-lang/pub#3618, since that is now fixed upstream, and removing it now avoids needing to update the workaround in every follow-up PR.

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 and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[tarrinneal]

Seems like everything here is solid