Generate a default preview for widgets that don't have a previewImage.

Change-Id: I3a3c24543c925a50ab34fd3cbf7947544a5ac5c0
diff --git a/src/com/android/launcher2/Launcher.java b/src/com/android/launcher2/Launcher.java
index 94556be..dd93cbb 100644
--- a/src/com/android/launcher2/Launcher.java
+++ b/src/com/android/launcher2/Launcher.java
@@ -953,7 +953,7 @@
 
         // Calculate the grid spans needed to fit this widget
         CellLayout layout = (CellLayout) mWorkspace.getChildAt(cellInfo.screen);
-        int[] spans = layout.rectToCell(appWidgetInfo.minWidth, appWidgetInfo.minHeight);
+        int[] spans = layout.rectToCell(appWidgetInfo.minWidth, appWidgetInfo.minHeight, null);
 
         // Try finding open space on Launcher screen
         final int[] xy = mCellCoordinates;