fix(ngAnimate): callback detection should only use RAF when necessary

Callbacks are detected within the internals of ngAnimate whenever an
animation starts and ends. In order to allow the user to set callbacks
the callback detection needs to happen during the next tick. Prior to
this fix we used $$rAF to do the tick detection, however, with this
patch we intelligently use $$postDigest to do that for us and then
only issue a call to `$$rAF` if necessary.