fix(ngAnimate): defer DOM operations for changing classes to postDigest

When ngAnimate is used, it will defer changes to classes until postDigest.
Previously, AngularJS (when ngAnimate is not loaded) would always immediately
perform these DOM operations.

Now, even when the ngAnimate module is not used, if $rootScope is in the midst
of a digest, class manipulation is deferred. This helps reduce jank in
browsers such as IE11.

/CC @tbosch --- alternative/more general fix here

Closes #8234
Closes #9263
Let's leave this as is for now, as we have the same strategy/problem in the `ngAnimate` module, and we would like to keep the behavior in sync between those two.