2D Recents: polish the app -> recents transition

* Do not draw the header as part of the transition thumbnail
  since the header is already shown. Just let the thumbnail
  slot into the task view already drawn on screen.
* Shift the target rectangle by the header height so that
  the end state of the animation corresponds exactly to the
  task view layout.
* Remove unused local variable.

Bug: 32101881
Bug: 33752154
Test: Checked/screencasted recents -> app -> recents animations on local sw600dp device
Change-Id: I4c8ce52b378567114735e6bd582572124a2a354c