I found a 2 month old thread (didn't want to post there) referring to this link for a modded APK for Xfinity Stream ( http://199.175.53.21/xfinitystream450010.leanback.apk ). I did manage to get this to work on my 2015 Sony Bravia Android TV, but this is actually the older Xfinity TV app. While this seems to work, it acts quite buggy. I sometimes have issues navigating selections and moving too quickly will outright crash and close the app.
There was a user in the old thread I found that said he had APK's for Xfinity Stream, wondering if I might get lucky and some wonderful person out there might still have something? :)
Using a modified apk intended for Fire TV (not firestick) and other than a main menu glitch where the headers don't display properly and some difficulty with the initial set up, it works great on Sony and element android tvs
Note does NOT allow watching TV without paid xfinity account, just treats your 55 inch android tv like a huge tablet