Unload widget previews when launcher is not visible

Can save ~1-2MB of RAM

Bug #5104303

Change-Id: I992bec33052f4d2c201e528b5edf384d05960e71

src/com/android/launcher2/PagedViewGridLayout.java

@@ -68,6 +68,12 @@ public class PagedViewGridLayout extends GridLayout implements Page {
                 heightMeasureSpec);
     }
 
+    @Override
+    protected void onDetachedFromWindow() {
+        super.onDetachedFromWindow();
+        mOnLayoutListener = null;
+    }
+
     public void setOnLayoutListener(Runnable r) {
         mOnLayoutListener = r;
     }