Add tun interface / VPN service

[mikedawson]

Use case:

Users wants to use the mesh network with other apps (e.g. meshenger, kiwix, file sharing, etc). Other apps might not want to embed Meshrabiya into their own source code. There could also be an issue if more than one app with Meshrabiya embedded is operating at the same time.

Solution:

Add an Android VPN service that is accessible over a tunnel interface, so any app can use the network.