optimize wallpaper load,avoid show black wallpaper.

[Preconditions]
open auto-rotate

[Procedures]
1.enter Contacts app, and rotate 90 degrees to the right
2.press power key to lock screen,and unlock
3.rotare 90 degrees to the left and exit Contacts app
4.the wallpaper will be black first,then show the really wallpaper

packages/SystemUI/src/com/android/systemui/ImageWallpaper.java

@@ -236,7 +236,9 @@ public class ImageWallpaper extends WallpaperService {
                     Log.d(TAG, "Visibility changed to visible=" + visible);
                 }
                 mVisible = visible;
-                drawFrame();
+                if (visible) {
+                    drawFrame();
+                }
             }
         }