Using DiffUtil for calculating widget diff instead

of a custom implementation

This uses the same version as all-apps and avoids whole list refresh
when a widget entry is expanded for the first time

Removing tests which were essentially testing the diffing logic

Bug: 268646258
Test: Verified on device
Change-Id: I4799c66b4674e754547317317e7a63f098fce735

src/com/android/launcher3/widget/model/WidgetListSpaceEntry.java

@@ -33,9 +33,4 @@ public class WidgetListSpaceEntry extends WidgetsListBaseEntry {
                 Collections.EMPTY_LIST);
         mPkgItem.title = "";
     }
-
-    @Override
-    public int getRank() {
-        return RANK_WIDGETS_TOP_SPACE;
-    }
 }