Trebuchet: Fix TaskbarViewCallbacksFactory instantiation

Overrides.getObject() calls the constructor with context if the overlay
has a valid class, if empty it uses the constructor without args.
Considering this class is part of quickstep, we should support both
cases, since this is the fallback class anyway

Based on 93f44a4c

Change-Id: Ida3f5e056522871dc35ce48a5488ad28b6b0c2cc