fix($animate): ensure that class-based animations only consider the most recent DOM operations

Prior to this fix $animate would maintain a count of each time a class was
added and removed within $animate. With this fix, $animate instead only cares
about the most recent class times that the classes were added and/or removed
and it will only remember a total of one addClass or removeClass operation per
className.

```
// before
addClass    => 1
removeClass => 0
addClass    => 1
addClass    => 2
addClass    => 3
removeClass => 2
removeClass => 1
// since 1 > 0 then this will cause an addClass animation

// now
addClass    => 1
removeClass => 0
addClass    => 1
addClass    => 1
addClass    => 1
removeClass => 0
removeClass => -1
// since 1 < 0 then this will cause an removeClass animation
```

Closes #8946
```js
$animate.addClass(element, 'two'); // cache.two === 1, element doesn't have .two
$animate.addClass(element, 'two'); // cache.two === 1, element doesn't have .two
$animate.removeClass(element, 'two'); // cache.two === -1, element doesn't have .two
$rootScope.$digest(); // nothing happens, because we wanted to remove a class that the element didn't have
$animate.triggerReflow();
```