Prevent dim surface from flashing.

This fix keeps the dim surface below the highest shown layer. If
two shown layers were both dim it was ambiguous where the dim surface
would appear causing dialogs to first be dimmed and then flash when
the dim was put behind them.

Fixes bug 6497476.

Change-Id: I360cf2d23d58fc4c03edbbed16d79c08c29e48b9