[image_picker_android] Fix deprecation warnings by branching based on build version, and suppressing only when needed

[gmackall]

Fixes https://github.com/flutter/flutter/issues/121816.

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• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• [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 linked to 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 ///).
• [ ] 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.

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