[video_player] fix misvalue at `isPlaying` after unplug earphone

[miguelfs]

• [ ] add tests
• [ ] add Android platform channel code (if necessary)

---

This PR fixes the following open issue:

• https://github.com/flutter/flutter/issues/49081

This bug exists because there's no response from both FLTVideoPlayerPlugin and FLTVideoPlayer when a route change happens.

The proposed solution is to:

• add a route change notification from AVAudioSession;
• send the notification to the dart layer as a EventSink;
• then fire pause() from the dart layer.

Example app before this PR:

• when disconnecting the earphone, the video stops running, but the video_player state is maintained as isPlaying
• then, by tapping at a first time at the player, the isPlaying state goes from true to false;
• the "play" button is shown;
• then, by tapping a second time, the isPlaying state goes from false to true;
• the video reproduces again.

https://user-images.githubusercontent.com/10662585/171498096-f278b9ff-1bd5-481c-a690-550f586f7a4a.mov

Example app after this PR:

• when disconnecting the earphone, the video stops running, and the isPlaying state is set to false;
• notice that the "play" button is shown right away;
• then, by tapping at a first time at the player, the isPlaying state goes from false to true;
• the video reproduces again.

https://user-images.githubusercontent.com/10662585/171498791-b9582a8d-7278-4f54-9c8e-ef771e1c15ab.mov

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.
• [ ] 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.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [ ] 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.
• [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Hixie]

@miguelfs Thanks for your contribution! Will you be able to continue working on this?

[miguelfs]

Hi, @Hixie. Thanks for the feedback :)

I've been busy lately, so I don't think I'll be able to finish it soon :/

[stuartmorgan-g]

Since this is marked as a draft and hasn't been updated in several months I'm going to close it to clean out our review queue. Please don't hesitate to submit a new PR if you decide to revisit this. Thanks!