[connectivity-service] Support network over USB in developer mode

[Benjamin Zeller]

Currently the SDK has to use "adb forward" in order to connect and talk to the devices.
However it would be much more reliable if we could create a network over USB and talk
directly to it.
Probably even run a DHCP server on usb0 so we do not need to setup the interface ourselves
and provide a DBUS callback so the SDK can trigger the usbnet to be created over ADB.

Quotes from a conversation with Wellark on IRC:

<zbenjamin> Wellark: would be interesting for the SDK
<zbenjamin> Wellark: instead of adb forwarding the ports, which is totally unreliable, we could create a network with the phone and talk to it directly
<Wellark> zbenjamin: well, you file a bug
<Wellark> zbenjamin: then you tell my manager to get me to do the UI
<zbenjamin> Wellark: the idea would be that we setup the initial stuff with adb. And then switch over to usb network which is decoupled from the adb completely
<Wellark> zbenjamin: yep.
<Wellark> adb would only make one call over dbus
<zbenjamin> Wellark: having a dhcp running on the usb interface would make things super easy
<Wellark> com.ubuntu.connectivity.usbnet.Enable()
<Wellark> or something
<Wellark> that would kick up the usbnet
<Wellark> zbenjamin: yep. and zeroconf dns
<zbenjamin> yeah. And we could make ssh only listen on this interface instead of all, and enable wlan only if the user checks a box in the UI
<Wellark> so you don't have to deal with IP's
<zbenjamin> exactly