[DO NOT MERGE] [OPTIONAL REVIEWERS: AMC/ME] Mmtel port

DO NOT MERGE. This breaks Sprout since there are no changes in this pull request to the calls into these files (i.e. from main.cpp and stateful_proxy.cpp). This is being done as a separate piece of work.

This pull request covers the changes to callservices.h and callservices.cpp to use the new AS API. These files have also been renamed to reflect the fact they represent a single AS. This name is subject to change. Every reference to MMTel may be replaced by another name.

There are no changes to stateful_proxy in this pull request, despite the fact I've pulled out the redirect functions into PJUtils.

There are still two TODOs which are covered by a single TODO in the code.