Google Play has a payment mechanism in place for subscriptions. I think the same Developer Agreement logic applies there, as well. I suspect that if you plan to offer subscriptions through your Google-Play-distributed app, you have to do so through Google Play unless the subscription is for physical goods consumed outside the Google Play environment (I suppose, like a physical magazine).
Feel free to consult the Developer Agreement, though. It's not written in obscure legalese. It's pretty plainly-written.