Android TV has been available in an unofficial form on the powerful Raspberry Pi 3 for some time now, and the newest version, based on Android 7.1.2 (Nougat) is now available. The build comes courtesy of XDA Developers user tabp0le. This new version of Android TV works admirably on the development board, though there are a few bugs to work out at the moment. The newest version of Android TV requires a little bit of elbow grease to get up and running on the Raspberry Pi 3, and you'll need a PC with a distribution of desktop Linux around. It should be noted that this build is not Android TV per se, but is actually AOSP Android with compatibility for Android TV apps, including Kodi, the default Android TV shell, and even Google's special suite of GApps made for Android TV.
If you have a Raspberry Pi 3 and a Linux computer sitting around, you can try out this build, but don't expect a perfect experience; there are a lot of miscellaneous bugs, and both YouTube and DRM content are not working properly at this point, though YouTube can be played with third party apps. The installation process is not for the faint of heart, and you should be fairly comfortable with a command line interface before giving it a try. You'll also need to have the Android Debug Bridge set up. Essentially, you're going to be manually partitioning an SD card and filling up the partitions with Android system files. The goal is to have a mirror image of what the target Raspberry Pi's internal file system should look like. Once that step is done, you'll boot up the Raspberry Pi, connect it to the same network as the Linux computer, and manually flash the GApps package.
If you've gone through the procedure correctly, your Raspberry Pi will reboot itself a few times over, then it will boot up into Android, complete with Android TV GApps and app compatibility. From there, you can customize it to behave like a phone or a tablet, or you can choose to turn it into a full-on Android TV streaming and media center with apps like Kodi, BubbleUPnP, and Moonlight Game Streaming.