refactor($animate): use $animate.$$setClassImmediately to save code

> Extending a promise is problematic since if another promise is chained then the methods are lost.

Sure, but you could say "always do this before calling `.then` (or similar)" until we have a nicer way to do that (I think we want a better way to subclass promises used within the framework so that we don't have that issue with $http)