fix($animate): only block keyframes if a stagger is set to occur

Transitions must be blocked so that the initial CSS class can be applied
without triggering an animation. Keyframes do not need to be blocked since
animations are always triggered on the starting CSS class, however, if a
stagger animation is set to occur then all elements for index > 0 should
be blocked. This is to prevent the animation from occuring early on before
the stagger delay for the given element has passed.

With ngAnimate and keyframe animations, IE10 and Safari will render a slight
flicker effect caused by the blocking. This fix resolves this issue.

Closes #4225

View broken effect (Safari || IE10):
http://bit.ly/keyframe-fail

View fixed effect (Safari || IE10):
http://bit.ly/keyframe-success