I could be wrong but the only app that can send and receive messages through RCS on Android is Googles own messages app. Since Google runs their own RCS instance, they're the only ones interfacing with it. There's currently no API to allow for third party apps to make use of RCS.

That being said, RCS is designed to be an open standard. It's just that only Google is really pushing for it right now and running an instance of it. If I'm not mistaken, AT&T ran their own instance for a while but it was shut down in favor of Googles instance.