fix($animate): insert elements at the start of the parent container instead of at the end

With 1.2.x, `$animate.enter` and `$animate.move` both insert the element at the end of the provided
parent container element when only the `parent` element is provided. If an `after` element is provided
then they will place the inserted element after that one. This works fine, but there is no way to
place an item at the top of the provided parent container using these two APIs.

With this change, if the `after` argument is not specified for either `$animate.enter` or `$animate.move`,
the new child element will be inserted into the first position of the parent container element.

Closes #4934
Closes #6275

BREAKING CHANGE: $animate will no longer default the after parameter to the last element of the parent
container. Instead, when after is not specified, the new element will be inserted as the first child of
the parent container.

To update existing code, change all instances of `$animate.enter()` or `$animate.move()` from:

`$animate.enter(element, parent);`

to:

`$animate.enter(element, parent, angular.element(parent[0].lastChild));`