Improved the animation logic of the stack scroller.

Newly introduced appear and disappear animations when in the shade.
Also introduced individual child delays such that notifications
appear in a slightly more appealing quantum way.
Also fixed a racecondition, such that added notifications already
have their final visibility state when they are added to the scroller.

Bug: 14081264
Change-Id: I18f5c57c2206f8e05996253981f540e97521e102