Initial changes to stack layout to support paging and nonsquare thumbs.

- Moving to a couple piecewise curves to define the various overview
  layout states.  Added a new state for focus (to be used in follow up
  CL) to control paging of overview from the nav bar button.  This 
  allows us to control the visible range of items on the curve, and 
  to better fit other UI controls around the stack.
- Removed the scaling of the tasks in the stack
- Also refactoring parametric curve to just use the system Path

Change-Id: I4108da77986d86896576e36fa8f31189d6fcb6f3