feat(ngAnimate): provide ng-[event]-prepare class for structural animations

The new prepare class is added before the animation is pushed to the
queue and removed before the animation runs, i.e. it is immediately
available when a structural animation (enter, leave, move)
is initialized.

The class can be used to apply CSS to explicitly hide these elements
to prevent a flash of content before the animation runs.
This can happen if a structural animation (such as ng-if) sits at the
bottom of a tree which has ng-class animations on the parents.
Because child animations are spaced out with requestAnimationFrame,
the ng-enter class might not be applied in time, so the ng.if element is
briefly visible before its animation starts.