Support media files

This is the first step in supporting adding artibitrary files to fdroid repos, targeted add supporting media files and flashable update.zip files like used to install the Privileged Extension. This reuses the existing metadata fields to keep compatibilty with older versions, but that means that lots of names are confusingly named since they refer to aspects of an APK rather than a generic file. We can address that later when we refactor the whole index metadata.