Refactor extended provider builder to simply build advertisements

[masih]

Right now it is possible for extended provider builder API to be used for building regular ads with redundant entries. For example, if there are no extended provides added to the builder, it always generates ads with extended providers that have duplicate extended provider info as the root.

We can avoid all of this, by refactoring the code, such that there is just ad builder that does the right thing when extended providers are added. That also makes it easier for clients to use the API.