I am about to give my phone for repairs (broken screen). I wonder, what if repair guy will reinstall OS with version, containing some spyware or such? Is it possible to detect this? I can encrypt the phone and lock it with the password, would it help? But what if he requires access to the phone during repair process (say, to check that it works properly after screen replacement)? In this case I can do factory reset to prevent sharing my personal data (it is backed up), but in this case how can be sure that OS wasn't tampered with?