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.
Merge branch 'feature/Fail-closed-prevent-leakage-when-VPN-is-down-#2023' into develop