Tweaking layout to make two-task stacks to closer match design.

- In the small-stack cases, set the initial state to top-align the
  back-most task.  In addition to offsetting the position, ensure that
  the dim for the back-most task is offset by this new position.
- Fixing issue with wrong return value when update insets.
- Fixing issue where tasks disappear after rotating while scrolling the
  stack, due to the scrollering setting the stack scroll to an invalid
  scroll position in the new layout in the new orientation.

Bug: 27859654
Change-Id: Ia4aaa3fe445032493c94a8dee7f6fea33bee36ec