service: Add Mock support for Daemon and CoreStack

This patch turns the Daemon and CoreStack classes into abstract
interfaces. Mock and real implementations are implemented in special
subclasses and obtained via a factory method. Other test implementations
(e.g. stub) can be provided in the future.

Bug: 22532180
Change-Id: Ic3000518e5cef1c867dd803cd0bba6aab49dc596