WgQuickBackend: by default use single-tunnel mode like GoBackend, but add option

Note that this currently doesn't play well with people activating
wg-quick tunnels from outside the app. Those tunnels won't be
deactivated. But presumably that's desired behavior anyway, considering
people are mucking around at the command line.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

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

@@ -104,6 +104,9 @@
     <string name="module_installer_working">Downloading and installing…</string>
     <string name="module_installer_error">Something went wrong. Please try again</string>
     <string name="mtu">MTU</string>
+    <string name="multiple_tunnels_title">Allow multiple simultaneous tunnels</string>
+    <string name="multiple_tunnels_summary_on">Multiple tunnels may be turned on simultaneously</string>
+    <string name="multiple_tunnels_summary_off">Turning on one tunnel will turn off others</string>
     <string name="name">Name</string>
     <string name="no_config_error">Trying to bring up a tunnel with no config</string>
     <string name="no_configs_error">No configurations found</string>

ui/src/main/res/xml/preferences.xml

@@ -8,6 +8,12 @@
         android:title="@string/restore_on_boot_title" />
     <com.wireguard.android.preference.ModuleDownloaderPreference android:key="module_downloader" />
     <com.wireguard.android.preference.ToolsInstallerPreference android:key="tools_installer" />
+    <CheckBoxPreference
+        android:defaultValue="false"
+        android:key="multiple_tunnels"
+        android:summaryOff="@string/multiple_tunnels_summary_off"
+        android:summaryOn="@string/multiple_tunnels_summary_on"
+        android:title="@string/multiple_tunnels_title" />
     <com.wireguard.android.preference.ZipExporterPreference />
     <com.wireguard.android.preference.LogExporterPreference />
     <CheckBoxPreference