fix(ngAnimate): run structural animations with cancelled out class changes

When multiple animations on the same element are queued before a $digest passes,
the animator tries to create as few actual animations as possible by joining / canceling
redundant animations. Class-based animations for example are cancelled when the classes that
are added and removed are the same, and the result is no class-change. This however must only
happen if there's no structural animation currently queued.

Fixes #14249