Expose CoreProcessStateService functionality used by subsystems via a capability

Subsystems are injecting CoreProcessStateService to track the process state, and perhaps more should as some are using graceful startup as a kind of trigger for beginning work, when the process reaching RUNNING state would be more appropriate. But:

1) they aren't using a capability for this wiring, because the kernel doesn't expose one
2) ControlledProcessStateService is not a clean API, as it exposes the MSC Service interface which should not be accessible to subsystems.

So, add a ProcessStateNotifier interface, and expose it via a capability. Impl is still ControlledProcessStateService.