There's no need to do the work that OnlineFileRequest does on a separate thread from the DefaultFileSource thread, and having AsyncTasks proxy to other tasks across a thread boundary adds needless complexity.
* 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.
