fix(ngAnimate): do not trigger animations if the document is hidden

Prior to this fix, ngAnimate would always trigger animations even if
the browser tab or browser window was not visible. This would cause
issues with class updates / DOM operations even if elements were not
using animations. The root cause is that browsers do not flush calls to
requestAnimationFrame when browser windows / tabs are not visible.

This fix disables animations if `document.hidden` is `true`.

Closes #12842
Closes #13776