As far as I know it is not possible to access files from the assets with the Activity starter. You have to copy them first to the external storage (best: ASD, because no WRITE permission is needed) and access them from there.
The Blocks with /.../Makeroid/assets/... will work with Companion, because these files are already in the external storage. As @Mohamed_Tamer said, the assets are in the internal storage (appPackage) after compiling the app (APK).