Implement VPN over native AP

mobile/src/main/res/values/strings.xml

@@ -5,4 +5,5 @@
         <item quantity="one">1 connected device</item>
         <item quantity="other">%d connected devices</item>
     </plurals>
+    <string name="exception_interface_not_found">Fatal: Downstream interface not found</string>
 </resources>

mobile/src/main/res/xml/pref_settings.xml

@@ -8,6 +8,11 @@
             android:title="Upstream interface"
             android:summary="%s"
             android:defaultValue="tun0"/>
+        <be.mygod.vpnhotspot.preference.AlwaysAutoCompleteEditTextPreference
+            android:key="service.wifi"
+            android:title="Wi-Fi interface"
+            android:summary="%s"
+            android:defaultValue="wlan0"/>
         <AutoSummaryEditTextPreference
             android:key="service.dns"
             android:title="Downstream DNS server:port"