When a phone is being stolen, the first thing a thief will try to do is to turn on flying mode or shutdown. Google play has an app called "Cerberus" that disables powering off and control panel on your phone when locked, but it asks for too many permissions for a proprietary app IMO. Does anyone know any F-Droid alternatives?
Plus isn't your phone's encryption at its strongest state after its been rebooted, before it's been unlocked? Guess it'll be useless unless OP has an outdated security patch, a weak passcode or if it was snatched while unlocked