Fixed issue with resized stack sticking to the top of the screen.

WindowState.mUnderStatusBar wasn't set correctly when the stack was
resized before the window was added to it. Changed to use
TaskStack.mUnderStatusBar instead which will always have the
correct answer.

Bug: 19204013
Change-Id: Ib4660302c6a2aebd47bff354c1efab93a1e1a255