platform: avoid conflicts when reinstalling the device-route

Since f32075d2fc11252e5661166b2f46c18c017929e9, we remove the kernel
added IPv4 device route, and re-add it with appropriate metric.

This could potentially replace existing, conflicting routes. Be more
careful and only take any action when we don't have a conflicting
route and when we add the address for the first time.

The motivation for this was libreswan which might install a VPN route
for a subnet that we also have configured on an interface. But the route
conflict could happen easily for other reasons, for example if you
configure a conflicting route manually.

Don't replace the device route if we have any indication that
a conflict could arise.

https://bugzilla.gnome.org/show_bug.cgi?id=723178