Obtain DNS server automatically from VPN service

Demote DNS settings to fallback usages only.

mobile/src/main/java/be/mygod/vpnhotspot/App.kt

@@ -28,7 +28,6 @@ class App : Application() {
 
     val handler = Handler()
     val pref: SharedPreferences by lazy { PreferenceManager.getDefaultSharedPreferences(this) }
-    val dns: String get() = pref.getString("service.dns", "8.8.8.8")
 
     fun toast(@StringRes resId: Int) = handler.post { Toast.makeText(this, resId, Toast.LENGTH_SHORT).show() }
 }