Make DimLayer track changes to WindowState layer.

The DimLayer behind popups was not changing when the popup layer
changed. It will now.

Fixes bug 7974415.

Change-Id: Ia486efa83d623716a09d73a22493a4222823c573
diff --git a/services/java/com/android/server/wm/DimLayer.java b/services/java/com/android/server/wm/DimLayer.java
index 4c7add9..88efe2e 100644
--- a/services/java/com/android/server/wm/DimLayer.java
+++ b/services/java/com/android/server/wm/DimLayer.java
@@ -84,6 +84,17 @@
         return mTargetAlpha;
     }
 
+    void setLayer(int layer) {
+        if (mLayer != layer) {
+            mLayer = layer;
+            mDimSurface.setLayer(layer);
+        }
+    }
+
+    int getLayer() {
+        return mLayer;
+    }
+
     private void setAlpha(float alpha) {
         if (mAlpha != alpha) {
             if (DEBUG) Slog.v(TAG, "setAlpha alpha=" + alpha);