Minor refactoring: consider moving EngineController out of MockListenerController

[nikitavlaev]

Description

Currently, there is a design-related problem with MockListenerController.
It requires EngineController, but Mocker is meant to be used even without existence of an engine.
For now, Mocker has an optional field of MockListenerController?, but that is still a dependency.

Expected behavior

Mocker should be independent of engine.

Context

The proposed solution is to pass EngineController directly to MockListeners when attached through engine.
This can be done through mutable nullable property.