[google_sign_in] Add forceCodeForRefreshToken parameter platform implementations

[fbcouch]

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This is the platform implementation companion to https://github.com/flutter/plugins/pull/3356.

Related Issues

• flutter/flutter#17813
• flutter/flutter#14245
• flutter/flutter#15796
• flutter/flutter#45847

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/plugins repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [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.
• [x] 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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan-g]

It looks like several of the new unit tests are failing; see https://cirrus-ci.com/task/5986776966758400 for details.

[fbcouch]

@stuartmorgan My bad – for whatever reason, Android Studio was loading the tests from the pub version of the plugin instead of the local branch...those are fixed now, and should be using the same style as the other init tests that were added.