Support for new multi-endpoint APIs.

- piping through "pullExternalCall" API from incall layer to connection
service.
- mapping of new capabilities/properties between connections/calls.
- basic unit tests for new pullExternalCall API (there will be some
follow-up work on these in the future).
- plumbing through of Connection and Call events.

Bug: 27458894
Change-Id: I421ebab28fada224bddca54ed4d3d9dff6f33bcf