I'm not sure what you mean? It's not going to work as a 3rd party service. So Apple can do it for its customers and I guess Google for its own, and Microsoft, and so on....

But how is Google going to ensure "full portability" across the lineup of all Android devices in the same way that Apple could do so for iOS?

Edit: Maybe this is what you meant? https://news.ycombinator.com/item?id=15234615