Fix hit targets, hopefully once and for all

When already in split screen we use the stage bounds from split screen
controller, however, these bounds really represent the draw region and
would be inset by taskbar etc. For the hit targets we really want the
whole side of the screen from the middle of the divider, rather than
the draw region.

This CL updates it so that the hit region is based of the display region
divided wherever the divider is.

Test: manual - have apps in split, drag a new one into split and observe
               that the animation is smooth
Bug: 207011867
Change-Id: I7142755febd7fe2f6c2db4cee2e08fa2f43c3b7f