monitor-manager: Ensure (privacy) monitors settings after backend has been updated

The monitors settings such as the privacy screen property is propagated to the monitors via kms updates, however during initialization and on monitors changes, we end up clearing the pending KMS updates because such settings are added to the queue before the backend has fully initialized the monitors, and this may lead to discarding all the pending updates, including the one we've just planned.

To avoid this, move settings applications after we've both initialized the backend and notified it about changes.