I want to reaffirm what was said. Do not change anything using sqlite or any other method. Xposed and the hdxposed plugin work to allow installs from other sources.
Very easy.
I'm sure you all know that Amazon locked down the ability to install apps from "Unknown Sources", which is why the only current method of sideloading apps is to do so over ADB. The following will let you enable "Unknown Sources", by directly changing the entry in /data/data/com.android.providers.settings/databases/settings.db, so you can perform app installations directly on the FTV (which will come in very handy for installed Xposed modules and other things).
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