[video_player] Build an empty view when the player is already disposed

[wilyan09007]

buildViewWithOptions resolves the player through _playerWith, which throws a StateError when the id is no longer present in _players. Disposal and the next frame are not synchronized, so a widget still holding a disposed player can be rebuilt during an AnimatedSwitcher transition, or by a rebuild that a configuration change triggers once the platform has reclaimed the player. Because this runs inside build, the throw leaves the video view permanently broken rather than simply rendering nothing.

Both platform implementations now look the player up directly and build an empty view when it is gone.

Fixes flutter/flutter#190739

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[gemini-code-assist]

Code Review

This pull request updates both the Android and AVFoundation implementations of the video_player plugin to return an empty view (SizedBox.shrink()) instead of throwing a StateError when building a view for a disposed player. It also includes corresponding unit tests and version bumps for both packages. There are no review comments, and I have no feedback to provide.