[camerax] Update instance manager and replace Java 8 language with Java 7

[camsim99]

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Replaces the instance manager with the updated on from @bparrishMines's https://github.com/bparrishMines/plugins/tree/wrapper_example/packages/wrapper_example and fixes a casting issue that requires Java 8, causing a problem internally.

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.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] 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.
• [x] 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.

[auto-submit[bot]]

auto label is removed for flutter/packages, pr: 3318, due to This PR has not met approval requirements for merging. Changes were requested by {reidbaker}, please make the needed changes and resubmit this PR. You have project association MEMBER and need 0 more review(s) in order to merge this PR.

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[auto-submit[bot]]

auto label is removed for flutter/packages, pr: 3318, due to - This pull request has changes requested by @reidbaker. Please resolve those before re-applying the label.