Add window to child window list with correct order

The original logic of comparing with mBaseLayer of two child windows
of the same parent was wrong, because the mBaseLayer is always the
same.

After adding two child windows with the same positive sublayer, the
one added later should have greater z-order. The order should be
consistent with WindowManagerService.addAttachedWindowToListLocked()

Change-Id: I789179aee61d141fad86bd24262fabfb1e517e66