Fixed.
Merge branch 'bug/NoSuchElementException-import-in-debug-build' into develop
Persistent tun activated.

Now the "Waiting for server reply" notification is shown along with
native Android's VPN notification. Next step: fix it.
Show a special notification while traffic is blocked.

When the user has already been connected to a VPN and she configured the
client to use persistent-tun, we should warn her that until the VPN
connection is again established, no traffic will flow.