Juwang-Add RunningTask event handler, bridge classes and methods

When running task event is received from an Evaluator, it will trigger the call at RunningTask Event Handler registered at Java side. We would like to propagate the event to C# cross the bridge so that C# client can have their RunningTask event handler so that to get the task information including the Active Context. 

This PR adds a Java RunningTask event handler, creates bridge classes and methods, handles the calls between Java and C#. 
we arleady have a check for runningTaskHandler == 0 above, let's handle it there.