That's very annoying, indeed. If you want to backup the Sony DRM keys before unlocking the phone (they will be lost after unlocking), do it before upgrading to Android 7. What is amazing is that all needed tools were available for desktop linux. The DRM backup (backupTA based on DirtyCow!) and flash tool. I used Android SDK tools to unlock it using Sony's instructions and then install TWRP and a custom ROM. I'm happy because I don't have a Windows PC available.