Don't set replacing flag in starting windows.

When ever an app is changing stacks we set replacing flag on all it's
windows. Starting windows can be part of an apps window list, but is
added by window manager not the app. So, when we set replacing flag
on the starting window it can cause the starting window never to be
cleaned up since we are expecting the client to replace it which will
never happen since it was added by window manager.
We shouldn't be setting replacing flag on starting windows since they
can never be replaced.

Bug: 26294740
Change-Id: I0a4f1e44188e96e73614130cbea02a3860850f58