An apk is an installation package for an app (it's actually just a zip file with a different extension - any zip file manager will open it - but not install it, only the package manager will). All versions of Android will always continue to support apks. Whether an apk is written that needs APIs not in 8.0 is asking for a prediction of the future, no one can do that.
But if you're talking about an Android TV box, you should probably be running Kodi to watch TV, movies, etc., so all you need to worry about is whether the box will support Kodi. And as of 18 Leia, it's supported by Android 7 (and probably, but since I haven't tried it, I can't say for certain, even earlier versions). Kodi isn't anything that takes advantage of the latest code in the latest version, it's like a horse-drawn wagon - a fantastic idea when it was invented, but basically unchanged for centuries. (It also runs on Windows, MacOS, iOS, Linux ... and maybe other operating systems.)