Disable "Unkown sources" setting in Android Oreo
In previous versions of Android, one could control the system-wide app installation from unknown sources, which saved time but was risky. Having refined control over settings makes your device more secure.
Moreover, the mechanism has now been shifted to app-specific instead of system-wide, that is you can now control app installation from particular apps. In order to modify that, head to settings and search for 'Unknown Sources', and you'll find Special App Permissions. In order to always allow a specific app to install apps, toggle it 'allowed'.