[video_player] Document how to use pigeon and update to the latest version

[Hixie]

This mainly splits out the test logic from the messages.dart file.

This is the interface-level change. See #3281 for the complete PR.

Pre-launch Checklist

• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• 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 Flutter Style Guide and the C++, Objective-C, Java style guides.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I signed the CLA.
• All existing and new tests are passing.

[Hixie]

Ok, flutter/packages#265 has an update to pigeon which should fix the failure here.

[Hixie]

(going to create this afresh)