Fixing task view heights in paging tasks

- Now, all task views will be bounded by the stack rect, and the 
  thumbnail bitmaps will be scaled accordingly to fit either by width
  (when stacked) or to the view rect (when freeform)
- Fixing issue where the history button was not offset in freeform
- Tweaking thumbnail sizes of fullscreen screenshots
- Still requires changes to fix clipping to the correct aspect ratio in
  freeform.

Change-Id: I678b87c2f06947d32f3bb7c60a35f28eb36b5a68