Bug 1089615 - Add filtered class soonest to avoid black-white flash

I cant think of a meaningful test for this. We could assert the class is there before showing the task manager, but that's not really the point - the outcome we want to assert on is that it doesnt flash black then white. 
and looks like we have the plumbing in place to add a quick test inspecting the detail of the cardviewbeforeshow event :)