This app was mentioned in 3 comments, with an average of 4.00 upvotes
There was a similar mirror based on a nexus 7 a few months ago by /u/hannahmitt . project is here: https://github.com/HannahMitt/HomeMirror
I don't know if you can buy them anywhere though... But if you don't know how to program, you can use her app: https://play.google.com/store/apps/details?id=com.morristaedt.mirror and make it with a tablet
i like the idea of the fire stick. i plan on using my old Nexus 7 (2012) for mine.
my hope is to make it removable from the side of the mirror instead of permanently affixing it to the mirror surface.
there is an app for it on both Github and the appstore for it. super simple and much slimmer than having a full monitor or TV, and a Rasperry pi to build into the frame. there is also the fact that i happen to have the tablet just laying there collecting dust at the moment. just need to buy the mirror and make a frame.
Oh and since reading through your messages it sounds like you were going to make the app from scratch, here is a generic framework that you can use instead. There is a github project you can fork.
https://play.google.com/store/apps/details?id=com.morristaedt.mirror&hl=en