If the suspect had a screen lock, would that create a reasonable expectation of privacy?

Would you have a problem with an officer opening a suspect's mail at the scene of an incident? Ripping paper is just as little effort as hitting buttons on a phone.

De facto privacy is very different from this legal concept of "expectation of privacy". Obviously a man with a gun can violate your de facto privacy easily. The question is, what boundary are you going to put on the legitimacy of that activity?

We don't require warrants to violate your de facto privacy - we require them to violate your legal expectation of privacy.