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 addClass or removeClass operation and will only perform
that operation (depending on what was last called).

```
// before
addClass    => +1
removeClass => 0
addClass    => +1
addClass    => +2
removeClass => +1
// this will cause an addClass animation

// now
addClass    => add
removeClass => remove
addClass    => add
addClass    => add
removeClass => remove
// this will cause a rmeoveClass animation
```

Closes #8946