If USBGuard is running, follow default behaviour
The default value for USBGuard´s ImplicitPolicyTarget
is block
. This change adheres to the principle of least surprise: GNOME should not default to allowing a USB device over the USBGuard configuration. If a user has installed USBGuard, it seems sensible to assume they want safe defaults: if not, they will have configured USBGuard to behave otherwise.