These tests are intended to reflect how someone using the custom layer feature might actually write their callbacks. Therefore the callbacks should not use mbgl-specific classes.
This reverts commit 69112a2a09.
* 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.
