This app was mentioned in 3 comments, with an average of 1.67 upvotes
Correct. There's already just the plain 'Showtime' app for Android TV if you want to pay for Showtime without a cable subscription: https://play.google.com/store/apps/details?id=com.showtime.standalone&hl=en
Yes, this (OPs post) is like HBOGO. Showtime has had their standalone service on Android TV for a while now. It has a 30 day trial as well.
Showtime has an app for Android and probably one for iThings that will allow you to buy a monthly subscription to Showtime without having subscribed to the cable channel. This is probably what I'll be doing to watch TP when it comes out.