If you don’t mind dealing with a bit of instability, you can now install an unofficial version of Android TV on a Raspberry Pi 3.
This is still a super early build, so you won’t have access to the Google Play Store, but you can sideload apps. The performance of those apps isn’t consistent, but a number of them seem to do well.
For example, Kodi, as well as a number of game emulators, don’t appear to have any major issues. This is still very much an early build and not a complete operating system by any means, but if you don’t mind messing around with concepts, Android TV is worth a crack. The video above walks you through the full installation process.