Router Service's `currentURL` does not include `rootURL`

I think this is basically re-opening #16851

The issue is that `currentURL` does not include the `rootURL`, where as the other APIs added by the router service RFC (such as `urlFor`) are explicitly spec-ed to include it.

Given that the motivating use case for the router service is to re-build ``, it seems not very likely that it would be correct to use `currentURL` without `rootURL` (checking if a link is active maybe?). It is also a bit strange to assume that the RFC would disagree with itself on what "URL" means without calling it out.

To investigate:

* Did the RFC intend the current behavior? Are there any good use cases for it?
* Can we still fix it? Is it too late?
* What do we do? Deprecate and come up with a new name?