fix(ngAnimate): properly cancel-out previously running class-based animations

Prior to this fix the addition and removal of a CSS class via
ngAnimate would cause flicker effects because $animate was unable
to keep track of the CSS classes once they were applied to the
element. This fix ensures that ngAnimate always keeps a reference
to the classes in the currently running animation so that cancelling
works accordingly.

The commit also adds a test for a previously untested animation merge path.

Closes #10156
Closes #13822