Cleaning up stack layout.

- Refactoring the layout to remove unused (and confusing) variables, 
  reducing the number of calculations when updating min/max bounds, and
  allow more accurate tracking of stack positions (for attached views).
- Tweaking the transform for single-task stacks so that they behave 
  better.
- Removing old, unused dismiss-all ui.

Change-Id: I8ec3bf062b0451543bd8a297aa810405f68c84cb