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