Improving transition from paging to stack.

- When we start scrolling, project the tasks onto the unfocused curve,
  then reduce the offsets from the projected task indices back to the
  normal indices as you scroll.  This doesn’t give you a perfect result,
  especially when scrolling in the same direction as the tasks are 
  offset, but is better than what we have now.

Change-Id: I055b08257fe1427e00e26ffa02f261cf51a8a2e0