This reverses #3095. Rationale:
* We're now exposing source attributes as a public API. Making those attributes unavailable at certain times complicates that API.
* We're preparing to split RenderSource out of Source. Removing this removes a point of coupling between the two.
* Move asset:// URL handling to DefaultFileSource.
* AssetFileSource implements FileSource interface and follows familiar implementation patterns.
* Move default implementation to platform/default, zip implementation to platform/android.
* Don't bother with modified / expires / etag -- assets are not cached so it doesn't matter.
* Don't bother with interleaving individual IO calls on the implementation thread. That adds a lot of complexity for very little benefit.
