fix(ngAnimate): always apply a preparation reflow for CSS-based animations

It's unpredictable sometimes to ensure that a browser triggers a reflow
for an animation. Prior to this patch, reflows would be applied
carefully in between parent/child DOM structure, but that doesn't seem
to be enough for animations that contain complex CSS styling rules.

Closes #12553
Closes #12554
Closes #12267
Closes #12554