[network] Check configured interface, fallback to usb0 if it does not exist Fixes: JB#21324

If for some reason the configured network interface does not exist. (As
rndis0 on mako/N4, while it is an android device, using the android gadget).
We test if the interface exists, if not we fall back on usb0.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>