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.
I regenerated assets.zip so that all file paths have an `assets/` prefix, as the Android AssetFileSource implementation asserts, and removed `TEST_DATA` from the paths.
* 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.
Annotation tiles may become partially parsed just like regular tiles,
for example if a point annotation is added to the map before the style's
sprite has been loaded. In such cases, they need to be reparsed or the
annotation will not be rendered. Previously, the code path for reparsing
would be short-circuited by a dynamic_cast<VectorTileData*> followed by
a null check. This commit removes that case and adds (back) a virtual
reparse method to the TileData interface.
