Sends the correct non-docked bounds to SystemUI

This CL fixes a bug that DisplayPolcy would send incorrect bounds to
SystemUI in the split-screen mode, because it would get the bounds from
the home stack, which is incorrect when the non-docked stack is not the
home stack.

Bug: 142390472
Test: Manually enter split-screen mode in landscape orientation, move
      the divider, and see if the icon color is correct.

Change-Id: Ic1a74ae814c677057085ad5de32bf3c8a7e19e86