Fix missing surface offset for stack with outset

This change restores the relative position calculation as
commit 32bcb10e (removed in commit ed6767fe).

Bug: 124532338
Test: atest TaskStackTests#testStackOutset
Test: manual - Observe there is no offset at the beginning
      of the pip enter animation. And when dragging the pip
      window to the left side and right side, the space
      between the edge of screen should be the same.

Change-Id: I05ca6aa6a3debf0a1f66a44d786fdd84348af3a4