Abstracts the details of the native->Dart communication system from the player instance by adding a listener protocol, and moving the event channel into an implementation of that protocol.

Also simplifies the logic to avoid messaging races during channel setup by adding a generic queue to the listener implementation instead of requiring the player to track whether or not it's allowed to send messages, and hard-coding specific messages to re-send later.

Part of flutter/flutter#172763

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^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.

### *Updated the comment refer to `StoreKit` instead of `Android`, in example/lib/main.dart*

### Before 
```dart
void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // When using the Android plugin directly it is mandatory to register
  // the plugin as default instance as part of initializing the app.
  InAppPurchaseStoreKitPlatform.registerPlatform();
```
### After
```dart
void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // When using the StoreKit plugin directly it is mandatory to register
  // the plugin as default instance as part of initializing the app.
  InAppPurchaseStoreKitPlatform.registerPlatform();
```

### Issue
Closes flutter/flutter#159623

## Pre-Review Checklist

[^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.

In preparation for marking `IOOverrides` as `base`.

Related to dart-lang/sdk#56468