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
@IgorMinar when here's why:

```
when VAL < 0 then it's removeClass
when VAL == 0 then nothing happens
when VAL > 0 then addClass
```

The idea here is that we can have one of three values. Using `Math.abs(VAL) < 1` can result in `true` or `false` but not an even value of `zero`. This wouldn't work with what we have here since we need to know when classes cancel each other out.