A server installs ServerService while an HC installs DomainModelControllerService, under different service names but both of which implement Service<ModelController>. To make it easier for subsystems that want ModelController access to work on both a server and an HC, we should create a capability with service type ModelController and have both processes use it.
Extensions may have a need to create an internal ModelControllerClient or to register notification handlers. Currently this is done by looking up the ModelController via its internal service name but:
1) ModelController exposes methods that should not be available to extensions
2) Accessing services should be via a capability.
So, we need to expose capabilities for these functions.