Fixing regression in stack task clipping.

- Removing expensive calls to map coordinates of front task to the tasks
  behind it, instead relying on the precalculated task rects that we
  already compute on each stack change update.  To prevent rounding
  issues, the task rects are now float rects.
- Also fixing crash when RecentsActivity starts before the SystemUI
  component (only an issue when pushing SysUI apks)

Change-Id: Ia84e9b9d165c0ce171c7fe3797e561ef24157a0a