launcher3: Introduce onGridMigrationComplete callback.

onGridMigrationComplete is invoked when grid migration
  successfully completes.

  ModelDelegate's can choose to observe this by overriding
  the callback.

Test: manual, existing
Flag: com.android.launcher3.one_grid_specs
Bug: 386802204

Change-Id: Iaca2e61a95c3d3decc12b2cc54461dba441202a0

tests/multivalentTests/src/com/android/launcher3/celllayout/FavoriteItemsTransaction.java

@@ -61,9 +61,11 @@ public class FavoriteItemsTransaction {
             ModelDbController controller = model.getModelDbController();
             // Migrate any previous data so that the DB state is correct
             if (Flags.gridMigrationRefactor()) {
-                controller.attemptMigrateDb(null /* restoreEventLogger */);
+                controller.attemptMigrateDb(
+                        null /* restoreEventLogger */, model.getModelDelegate());
             } else {
-                controller.tryMigrateDB(null /* restoreEventLogger */);
+                controller.tryMigrateDB(null /* restoreEventLogger */,
+                        model.getModelDelegate());
             }
 
             // Create DB again to load fresh data