[image_picker] Implement `getMedia` for picking videos and images simultaneously on Android.

[BeMacized]

This PR implements the getMedia method added to the platform interface in #4675 on Android, allowing users to pick both images and videos from the gallery simultaneously. It superses #4676 and will remain in draft mode until the changes to the platform interface have been merged and published, so that this PR may be updated.

Relevant issue:

• flutter/flutter#89159

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.

[Hixie]

@BeMacized Is this still on your radar?

[Hixie]

@BeMacized I'm going to close this for now but don't hesitate to reopen it or submit a new PR if you get back to it.