Avoiding intermediate states in NotificationStackScroller

The StackScrollAlgorithm was modified such that the notifications
now don't layout anymore during scrolling and therefore
intermediate states are avoided except for the first card.
Also made the top stack a bit smaller and fixed a bug where the
scrolling was not working on the very first try.

Bug: 14080821
Bug: 14081652
Change-Id: I924a9f8532486856fc2ecd88f6c10d26023a5bc3