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
question: why are we counting anything at all? `-1` -> class was removed last time, `+1` -> class was added --- this way you get the most recent result. It doesn't matter if the same classes was added N times or removed N times, only the most recent operation matters