The FTV doesn't have sqlite3 installed, or else this would be much simpler. These steps show you how to change the setting using the SQLite Editor app.
Note: Navigating the SQLite Editor app may require a mouse
- Root your FireTV according to the steps here
- Install SQLite Editor via ADB
- Launch SQLite Editor, granting it SU access
- Select "Settings Storage"
- Select "settings.db"
- Select the "global" table
- Highlight the record "install_non_market_apps"
- Hit the Menu button and select "Edit Record"
- Change the Value to "1" and save the record