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/WidgetsListHeaderEntry.java

@@ -85,12 +85,6 @@ public final class WidgetsListHeaderEntry extends WidgetsListBaseEntry {
         return "Header:" + mPkgItem.packageName + ":" + mWidgets.size();
     }
 
-    @Override
-    @Rank
-    public int getRank() {
-        return RANK_WIDGETS_LIST_HEADER;
-    }
-
     public boolean isSearchEntry() {
         return mIsSearchEntry;
     }