[video_player] Example App refactor

[ianko]

Description

Added a tab to include a live stream demo to the example app. This may help new changes to be tested against live streams as well.

The example app had, and still has, an issue when disposing videos from asset. While trying to investigate the issue, I decided to refactor and clean the example app for a better visibility.

Related Issues

No related issues but, the #1743 is needed in order to play a live stream.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[cyanglaz]

Waiting for #1743 (review) to land before starting reviewing this PR.

[cyanglaz]

I am adding this to backlog since #1743 is still in review.