How to natively automate an app with usb debugging disabled?
The Settings-App uses a persistent system property to enable/disable 
adb. 


The property_service inside  the system controls which "process-id" 
is allowed to set the different properties. Any Property starting 
with 
"persistent.service.*" can only be set by AID_SYSTEM. 

So you app must be a system app to be able to set this kind 
of properties and for this you have to sign it with the platform key 
of the particular device.