Android O is focusing on a wide variety of issues to make the user experience better and more secure. One feature, uncovered by Android Police, blocks apps installing APKs until the user explicitly approves that application. Users still need to allow installing apps from unknown sources, but it would then give more control over which apps these are. It’d prevent an app from maliciously installing malware even though other apps may be okay.
This is nice in general. Users get notified, open settings, an turn on this permission. However, this only exists on phones. On Android TV, there are currently no exposed settings UI to accomplish this. Will this UI be added in a future revision? Probably. For now though, apps like App Cloner and Tv App Repo will not be functional as both try to install APK files.
To demonstrate this concept I have loaded the Android O beta onto an emulator. In this video example, I have a downloaded APK. Normally it can be selected an installed using the Package Installer. On the O beta, we are alerted that this won’t work. Trying to select “Settings” does nothing, so we can’t approve the app’s installation.
The issue also extends to sideloading in other forms. Want to install an app by navigating a file browser? Unfortunately it’s using the same APIs to request the install and it will also fail.
If you’re really eager to sideload apps on your Android TV, you should probably refrain from signing up for the beta. If you don’t mind only getting apps from the Play Store for now, then go ahead explore the new UI.