Do not reset css position when using helper: clone

This is actually a work-around for issue with CSS left, top being cleared on drag stop. This is a problem if the draggable has absolute positioning and placeholder: 'keep' is used -- the draggable disappears on drag stop. In my situation I was using helper: clone anyway, so this patch just leaves the draggable's CSS positioning alone since it never changed in the first place.

This also restores the z-index fix of a few commits ago -- the recent refactoring broke that z-index fix. The problem is that mutateDraggable is called within a $timeout() delay, so the z-index fix restores the original z-index, and then the delayed mutateDraggable just blows it away again.