Fix DNS over mobile networks on Android

[johnmaguire]

I think this closes the loop on DNS issues I was experiencing.
Previously, after starting Nebula, DNS would work until you switched
networks (e.g. from mobile to WiFi or vice-versa). This was fixed by
removing some explicit DNS server sets in commit
a283bf8. This casued DNS to work in
adb shell even after toggling networks.

However, it did not actually fix the problem for Android applications.
The new behavior is that they would work while on WiFi, but fail on a
mobile network.

To quote Android docs:

Allows traffic from the specified address family. By default, if no
address, route or DNS server of a specific family (IPv4 or IPv6) is
added to this VPN, then all outgoing traffic of that family is blocked.
If any address, route or DNS server is added, that family is allowed.
This method allows an address family to be unblocked even without adding
an address, route or DNS server of that family. Traffic of that family
will then typically fall-through to the underlying network if it's
supported. family must be either AF_INET (for IPv4) or AF_INET6 (for
IPv6). IllegalArgumentException is thrown if it's neither.

In my case, my home network supports only IPv4 while my mobile network
uses DNS over IPv6. Since my Nebula routes are IPv4-only, IPv6 traffic
stopped working, and DNS requests failed.