Show an error message if VPN is not supported

Some devices have Android 4 installed but the system images still seem to
lack the components that are required for VPN support. One such
component is the dialog used to grant permission to create .

src/frontends/android/res/values-de/strings.xml

@@ -23,6 +23,8 @@
     <string name="reload_trusted_certs">CA-Zertifikate neu laden</string>
     <string name="show_log">Log anzeigen</string>
     <string name="search">Suchen</string>
+    <string name="vpn_not_supported_title">VPN nicht unterstützt</string>
+    <string name="vpn_not_supported">Ihr Gerät unterstützt keine VPN Anwendungen.\nBitte kontaktieren Sie den Hersteller.</string>
 
     <!-- Log view -->
     <string name="log_title">Log</string>

src/frontends/android/res/values/strings.xml

@@ -23,6 +23,8 @@
     <string name="reload_trusted_certs">Reload CA certificates</string>
     <string name="show_log">View log</string>
     <string name="search">Search</string>
+    <string name="vpn_not_supported_title">VPN not supported</string>
+    <string name="vpn_not_supported">Your device does not support VPN applications.\nPlease contact the manufacturer.</string>
 
     <!-- Log view -->
     <string name="log_title">Log</string>