Reparent children before reparenting to null

When we reparent a layer to null, we remove its child layers from current state and remove
their relative z. If the children are reparented in the same transaction, then we have to
make sure we reparent the children first so we do not lose their relative z order.

Test: atest SurfaceFlinger_test
Bug: 132205507
Change-Id: I6b73758065c7eb46ac3b60a1b40f6839be24b09c