Fixing custom widgets support:

> Moving the definitions to xml so that it is easier to override in derivative projects
> Fixing verious bind and save logic for custom widgets
> Adding feature flag to easily disable custom widgets

Change-Id: I0e278bc7dd415713029364060ef10842da990be9

src/com/android/launcher3/model/GridSizeMigrationTask.java

@@ -726,7 +726,7 @@ public class GridSizeMigrationTask {
                                 mContext).getLauncherAppWidgetInfo(widgetId);
                         Point spans = null;
                         if (pInfo != null) {
-                            spans = pInfo.getMinSpans(mIdp, mContext);
+                            spans = pInfo.getMinSpans();
                         }
                         if (spans != null) {
                             entry.minSpanX = spans.x > 0 ? spans.x : entry.spanX;