User defined wifi/usb tether network. [2/2]

Enables users to select from a list of predefined networks
for use as the tether network, overriding the built-in networks.
This is useful, for example, where OpenVPN is in use with
routed setup.

Change-Id: I0dce2379a3f85fc63e8140e0e8c9c4c8889c5ae2