VPN Hotspot 2.0: Client+ (#39)

Fix #13, #38. I don't have a lot of confidence that this would work very well for every device.

Also here's an SQL command that hopefully somebody could make into the app for me: `SELECT TrafficRecord.mac, SUM(TrafficRecord.sentPackets), SUM(TrafficRecord.sentBytes), SUM(TrafficRecord.receivedPackets), SUM(TrafficRecord.receivedBytes) FROM TrafficRecord LEFT JOIN TrafficRecord AS Next ON TrafficRecord.id = Next.previousId WHERE Next.id IS NULL GROUP BY TrafficRecord.mac;`

mobile/src/main/java/be/mygod/vpnhotspot/manage/TetheringFragment.kt

@@ -90,7 +90,7 @@ class TetheringFragment : Fragment(), ServiceConnection {
     var tetheringBinder: TetheringService.Binder? = null
     val adapter = ManagerAdapter()
     private val receiver = broadcastReceiver { _, intent ->
-        val extras = intent.extras!!
+        val extras = intent.extras ?: return@broadcastReceiver
         adapter.update(TetheringManager.getTetheredIfaces(extras),
                 TetheringManager.getLocalOnlyTetheredIfaces(extras),
                 extras.getStringArrayList(TetheringManager.EXTRA_ERRORED_TETHER)!!)