Implements milestone 1.1 of the [overlays implementation plan](https://docs.google.com/document/d/1EcP2GN8k8N74hHZyD0KTqm9oQo5-W1dZMqIVyqVGtlo/edit#bookmark=id.76uzcie1dg9d) PiperOrigin-RevId: 491696361
