fix($animate): wait two until two digests are over until enabling animations

Even when no remote templates are to be downloaded, wait two digests until
enabling animations since all structural digests perform a post digest before
running animations.

Closes #8844


Broken Example (no remote templates -> `snapshot`):
http://plnkr.co/edit/LREaTTieMJj4lOPJTjI0?p=preview

Working example (no remote templates -> `this fix`):
http://plnkr.co/edit/CuigpbMIB4ZkdmhWJ1bO?p=preview

Working example (remote templates -> `snapshot`)
http://plnkr.co/edit/QLD5TVtWnHGvCLnFcCYw?p=preview

Working example (remote templates -> `this fix`)
http://plnkr.co/edit/YtjFzsb7bR2AwfXD9lF2?p=preview
we need to wait until the end of current digest and execute after all the postDigest tasks (registered by enter, leave, move animations) scheduled during the digest have been flushed.