Draft: [camera] Add support for NV21 image format on android (part 2/3)

[acoutts]

This is part 2 of !6985 which adds support for NV21 image streaming on Android. It updates the deprecated legacy code path for image format groups and adds handling for the new NV21 option.

Note: we need #7021 to be merged and published first, then this change will be valid and the new unit test will pass.

List which issues are fixed by this PR. You must list at least one issue.

• [camera] Image stream data is corrupt on some Android devices (Vivo, Xiaomi, Motorola) flutter#118350

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.

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

[acoutts]

Note: this is marked draft until the update to camera_platform_interface is published from part 1, so we can update the new pub version for that here.

[acoutts]

cc @camsim99

[stuartmorgan-g]

Marking as draft until #6985 is reviewed.