Modify omemo_set_device_list to check against the persistent
known_devices.txt file before notifying the user. This prevents
false-positive security alerts on every restart for devices
that have already been registered and trusted.
Check against the persistent known_devices.txt file before notifying
the user. This ensures that "New OMEMO device" alerts are only shown
for devices that are truly new to this client.
Ref: ce9b5140d
b7c8f8235c