improving performance of first AllApps/Customize animation

Change-Id: I71e5cc634f7e1346617d101efd6945c00484cab3

src/com/android/launcher2/CellLayout.java

@@ -571,6 +571,10 @@ public class CellLayout extends ViewGroup {
         mChildren.draw(canvas);
     }
 
+    void buildChildrenLayer() {
+        mChildren.buildLayer();
+    }
+
     @Override
     protected void onAttachedToWindow() {
         super.onAttachedToWindow();