Fix bug #8196984 android.widget.cts.ToastTest failures

- Add registerLayoutListener() and assertLayoutDone(), because layout
  should be done before calling view.getLocationOnScreen()

- assertShowToast() checks whether the view is attached
  to view hierarchy. But it does not confirm that layout is done

- bug #7681540 and #7689906 are same issues to #8196984

Change-Id: I194241d12145ed947334dc51fe47ead732d794bd