interimElement: finish animation of previous element

[devversion]

The $$interimElement factory currently only supports single interimElements at the same time.

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This is intentionally, but it doesn't work properly for some components.

• For example, if you quickly open some toasts, the first toasts will overlay each other
  (See Toasts overlap #4822, $mdToast increases watchers but does not remove them if multiple toasts are displayed within a digest. #5193)

This is caused by the $$interimElement's item stack.

Once a second interim element is opening, we cancel the previous interim element and show the new one after the animation has started

This is not correct, because we should wait for the animation to be finished