[video_player_android] allow supplying a custom VideoAsset

[a1rwulf]

Adds VideoAssetProvider and VideoPlayerPlugin.setVideoAssetProvider, which is consulted before this plugin's own URI handling and may return null to fall through to it.

This is the extension point for playback the plugin cannot express itself: reading from a download or HTTP cache, applying a custom DataSource.Factory, or resolving a scheme the plugin does not know about. VideoAsset is already public and already exposes MediaItem and MediaSource.Factory, so this adds no new type surface; it only makes the existing one reachable.

Behavior is unchanged when no provider is registered, which the added tests cover alongside the override path.

This enables me to publish a companion package to allow downloading of HLS on Android and iOS.
Companion package source:
https://github.com/a1rwulf/video_player_offline

This is an attempt to upstream a fix for:
flutter/flutter#59759

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[gemini-code-assist]

Code Review

This pull request introduces VideoAssetProvider and VideoPlayerPlugin.setVideoAssetProvider to the Android video player plugin, enabling custom video asset resolution for URIs. It also adds unit tests to verify this behavior and bumps the package version to 2.13.0. The reviewer recommended marking the static videoAssetProvider field as volatile to ensure thread visibility and prevent stale reads.

[gemini-code-assist]

Since videoAssetProvider is a static field that can be set from any thread (e.g., during background initialization or dependency injection) and read on the main thread, it should be marked as volatile to ensure thread visibility and prevent stale reads.

Suggested change

	private static @Nullable VideoAssetProvider videoAssetProvider;
	private static volatile @Nullable VideoAssetProvider videoAssetProvider;