Add missing meta files + small update to prefab placement in AR demo scene

Assets/ARContent.meta

Assets/ARContent/ARScenePrefabs.meta

...ePrefabs/15dd32c9-ff00-428c-a4f2-95df6e9cb3c2.prefab.meta

...ePrefabs/269ae4e5-ff00-41f6-9eb3-011b8c87d158.prefab.meta

...ePrefabs/5ca5bb36-0001-44e5-a338-608ca8e7e787.prefab.meta

...ePrefabs/859d2682-ff00-420e-8601-362084de7a7e.prefab.meta

Assets/ARContent/ARScenePrefabs/Old.meta

...Old/5ca5bb36-0001-44e5-a338-608ca8e7e787_old2.prefab.meta

.../Old/5ca5bb36-ff00-44e5-a338-608ca8e7e787_old.prefab.meta

.../Old/be1f62b8-ff00-4f6c-9f5a-d48795494037_old.prefab.meta

...Old/be1f62b8-ff00-4f6c-9f5a-d48795494037_old2.prefab.meta

...fabs/Old/f3fd65e9-eb3e-4ea9-a35f-42187de1f79f.prefab.meta

...ARContent/ARScenePrefabs/ValidateAccessToBuilding.cs.meta

...Content/ARScenePrefabs/ValidateAccessToDataCenter.cs.meta

...ePrefabs/be1f62b8-ff00-4f6c-9f5a-d48795494037.prefab.meta

Assets/ARContent/Materials/Green 1.mat.meta

Assets/ARContent/Materials/Materials.meta

Assets/ARContent/Materials/Materials/anchor.mat.meta

Assets/ARContent/Materials/Materials/trackable.mat.meta

Assets/ARContent/Materials/anchor.png.meta