schemas: add privacy settings for USB protection
This is related to the GNOME-Control-Center MR gnome-control-center!366 (merged), the GNOME-Shell MR gnome-shell!369 and the gnome-settings-daemon MR gnome-settings-daemon!75 (merged).
If usb-protection key is true, it means that the user wants that GNOME handles the USB protection.
usb-protection-level controls when new USB devices should be blocked. One is "block when the lock screen is active" and two is "always block".