Modified to add event listener only when creating new candidate object

Modified to add event listener only when creating new candidate object

Since the previous create_softcandidate function adds event listener
every time it is called, which calls push_back() of the vector object,
resulting consume unnecessary memory. So changing the logic to add
event listener only when creating a new one.

Change-Id: I551a8bddeb222a80ea4a03732942f9f94e8ea16d