Don't throw when request size and docked stack is not visible.

When requesting size for the stack that is next to the dock stack, we
shouldn't crash if the dock stack is not visible (which we would treat
as if it was not availble). We continue to throw an exception if the
docked stack really isn't there, but if it's only not visible, we give
the other stack full screen bounds.

Bug: 25019182
Change-Id: Ia611733590a6cb5a4ef4cc636aa0a2a6a0958ceb