As an alternative to an Android TV, you could look into the Plasma Bigscreen project. Run it on whatever hardware you have available.
Plasma Bigscreen is an open-source user interface for TV's. Running on top of a Linux distribution, Plasma Bigscreen turns your TV or setup-box into a fully hackable device. A big launcher giving you easy access to any installed apps and skills. Controllable via voice or TV remote.
I have only found the source code for the Android and iOS application, but not for the server.