[video_player_avfoundation] Create a Dart player instance

[stuartmorgan-g]

Refactors the Dart internals to create a pleyer instance class on the Dart side, and intermediates the event stream with an instance-managed stream controller that would allow for alternate event sources (e.g., Dart code).

This prepares for future moves of more logic to Dart, and parallels Android Dart code changes made in
#9771

Part of flutter/flutter#172763

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
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• I updated/added any relevant documentation (doc comments with ///).
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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[stuartmorgan-g]

This was just an opportunistic change; I had mass-changed flutter.io to flutter.dev in most places a while ago, and apparently missed this channel name.

[gemini-code-assist]

Code Review

This pull request refactors the Dart implementation of the AVFoundation video player to introduce a _PlayerInstance class. This new class encapsulates the state and behavior for a single video player instance, including handling API calls and event streams. This is a good architectural improvement that increases modularity and maintainability. The event channel name has also been updated from flutter.io to flutter.dev for consistency. The tests have been updated to reflect these changes.

I've found a couple of issues related to the new StreamController in _PlayerInstance that could lead to resource leaks or runtime errors. Please see my detailed comments.