nm-vpn-service-plugin: increase the quit timer

nm-vpn-service-plugin: increase the quit timer

We now (since 3272ff6 libnm/libnm-glib: don't quit in the middle of asking for
secrets) always hook on the quit timer when NM asks the plugin if it needs
secrets. The timer is 20 seconds, which seems too short.

Let's make it three minutes. Don't bother adding another timer or using a
distinct timeout: it does no harm for the plugin to remain unused for three
minutes on a bus.

Another option would be to completely unhook it; however the plugin wouldn't
learn if the user cancelled the NM's secrets request and would remain unused
on the bus forever.