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/java/com/wireguard/android/Application.java

@@ -35,7 +35,7 @@ import java.util.Locale;
 
 import java9.util.concurrent.CompletableFuture;
 
-public class Application extends android.app.Application {
+public class Application extends android.app.Application implements SharedPreferences.OnSharedPreferenceChangeListener {
     private static final String TAG = "WireGuard/" + Application.class.getSimpleName();
     public static final String USER_AGENT;
 
@@ -86,7 +86,9 @@ public class Application extends android.app.Application {
                     try {
                         if (!didStartRootShell)
                             app.rootShell.start();
-                        backend = new WgQuickBackend(app.getApplicationContext(), app.rootShell, app.toolsInstaller);
+                        WgQuickBackend wgQuickBackend = new WgQuickBackend(app.getApplicationContext(), app.rootShell, app.toolsInstaller);
+                        wgQuickBackend.setMultipleTunnels(app.sharedPreferences.getBoolean("multiple_tunnels", false));
+                        backend = wgQuickBackend;
                     } catch (final Exception ignored) {
                     }
                 }
@@ -167,5 +169,19 @@ public class Application extends android.app.Application {
         tunnelManager.onCreate();
 
         asyncWorker.supplyAsync(Application::getBackend).thenAccept(futureBackend::complete);
+
+        sharedPreferences.registerOnSharedPreferenceChangeListener(this);
+    }
+
+    @Override
+    public void onTerminate() {
+        sharedPreferences.unregisterOnSharedPreferenceChangeListener(this);
+        super.onTerminate();
+    }
+
+    @Override
+    public void onSharedPreferenceChanged(final SharedPreferences sharedPreferences, final String key) {
+        if ("multiple_tunnels".equals(key) && backend != null && backend instanceof WgQuickBackend)
+            ((WgQuickBackend)backend).setMultipleTunnels(sharedPreferences.getBoolean(key, false));
     }
 }

ui/src/main/java/com/wireguard/android/activity/SettingsActivity.java

@@ -99,7 +99,8 @@ public class SettingsActivity extends ThemeChangeAwareActivity {
 
             final Preference wgQuickOnlyPrefs[] = {
                     getPreferenceManager().findPreference("tools_installer"),
-                    getPreferenceManager().findPreference("restore_on_boot")
+                    getPreferenceManager().findPreference("restore_on_boot"),
+                    getPreferenceManager().findPreference("multiple_tunnels")
             };
             for (final Preference pref : wgQuickOnlyPrefs)
                 pref.setVisible(false);

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