portal network monitor: Always emit changed signal on changed

The ::network-changed signal is documented to indicate any change in network configuration, which doesn't necessarily imply a property change - additional services becoming available after connecting to a VPN comes to mind for instance.

In order to match the "native" network monitor's behavior, always emit the signal when it's in response to the 'changed' D-Bus signal.

Also emit the signal unconditionally when loading the initial property values, to allow clients to differentiate between "offline" meaning "offline" and "offline" meaning "uninitialized".

Related: https://github.com/flatpak/xdg-desktop-portal/pull/221