Fixing animation spec animation problem.

- We were always using the stack layout to get the task transforms, but
  when we are animating to a given task position (ie. hitting recents 
  while the home->recents transition is happening), we would start the
  animation with the final task rect, instead of the current task rect.
  Instead, use the current transform of the task view, and transform
  that to screen coordinates and use it.

Bug: 27154882
Change-Id: I25af8fad97ee78669c952dd81708e6cee5dfab05