Add button to open exported videos in composition demo app

Enhances the composition demo application by allowing users to directly
launch exported videos after export completion.

- Configures FileProvider in the demo application manifest with
  external cache path access.
- Adds outputFilePath tracking to ExportState in the preview view model.
- Renders an interactive "Play Exported Video" button in ExportOptions
  using Intent.ACTION_VIEW when export completes successfully.

PiperOrigin-RevId: 941220368