This CL makes it possible to create a media3 ControllerInfo in test code, which is needed to test several aspects of a media3-based media app. It does this by exposing a test-only static factory method. This is a hacky low-effort approach; a better solution could be to split ControllerInfo up into a public interface that was exposed to client logic, and that they could extend, and a package-private implementation with internal fields like the callback. That's a much bigger change, however.
PiperOrigin-RevId: 491978830
