Converts google_sign_in_android and google_sign_in_ios to use internal method channels instead of the shared method channels. This was done by:

• Copying the default method channel, its supporting utils.dart file, and its test into each package, unchanged.
• Renaming the files and classes, and updating the test files to point to the right class.
• Renaming the channel itself to use a different name from the shared channel
• Adding the Dart registration logic (and tests for it), and bumping the minimum version to 2.8 where Dart registration for mobile was added.

I made a few extra changes:

• Added an integration test that at least validates that the platform channel is using the right name, since the existing test was a placeholder that didn't validate anything about the interaction with the native side.
• Did some low-hanging-fruit simplification of the iOS implementation:
  • Removed a no-op method that was only there for Android, no-oping it on the Dart side
  • Moved throwing on a sign-in type that is Android-only to the Dart side (keeping the same code and message for compatibility)
  • Added Dart unit tests of those changes.

Part of flutter/flutter#94224

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/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin 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.