Add DHCP workaround

Thanks to petergunn666 for debugging this with me.

mobile/src/main/res/drawable/ic_action_build.xml

@@ -0,0 +1,6 @@
+<vector android:autoMirrored="true" android:height="24dp"
+    android:viewportHeight="24.0" android:viewportWidth="24.0"
+    android:width="24dp" xmlns:android="http://schemas.android.com/apk/res/android"
+    android:tint="?attr/colorControlNormal">
+    <path android:fillColor="#FF000000" android:pathData="M22.7,19l-9.1,-9.1c0.9,-2.3 0.4,-5 -1.5,-6.9 -2,-2 -5,-2.4 -7.4,-1.3L9,6 6,9 1.6,4.7C0.4,7.1 0.9,10.1 2.9,12.1c1.9,1.9 4.6,2.4 6.9,1.5l9.1,9.1c0.4,0.4 1,0.4 1.4,0l2.3,-2.3c0.5,-0.4 0.5,-1.1 0.1,-1.4z"/>
+</vector>

mobile/src/main/res/values-zh-rCN/strings.xml

@@ -71,6 +71,8 @@
     <string name="settings_service_upstream_auto">自动检测系统 VPN</string>
     <string name="settings_service_clean">清理/重新应用路由规则</string>
     <string name="settings_service_clean_summary">将修改的设置应用到当前启用的服务上。也可用于修复偶尔会发生的竞态条件。</string>
+    <string name="settings_service_dhcp_workaround">尝试修复 DHCP</string>
+    <string name="settings_service_dhcp_workaround_summary">如果设备无法获取 IP 地址，尝试打开这个选项。</string>
     <string name="settings_misc">杂项</string>
     <string name="settings_misc_logcat">导出调试信息</string>
     <string name="settings_misc_logcat_summary">这种非常有用哇</string>

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

@@ -74,6 +74,8 @@
     <string name="settings_service_dns">Fallback DNS server[:port]</string>
     <string name="settings_service_upstream">Upstream network interface</string>
     <string name="settings_service_upstream_auto">Auto detect system VPN</string>
+    <string name="settings_service_dhcp_workaround">Enable DHCP workaround</string>
+    <string name="settings_service_dhcp_workaround_summary">Use this if clients cannot obtain IP addresses.</string>
     <string name="settings_service_clean">Clean/reapply routing rules</string>
     <string name="settings_service_clean_summary">Update changed settings to current active services. Can also fix rare
         race conditions.</string>

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

@@ -40,6 +40,11 @@
             android:summary="@string/settings_service_upstream_auto"
             android:hint="@string/settings_service_upstream_auto"
             android:singleLine="true"/>
+        <SwitchPreference
+            android:key="service.dhcpWorkaround"
+            android:icon="@drawable/ic_action_build"
+            android:title="@string/settings_service_dhcp_workaround"
+            android:summary="@string/settings_service_disable_ipv6_summary"/>
     </PreferenceCategory>
     <PreferenceCategory
         android:title="@string/settings_misc">