[Android] fix client registration in ConduitService using IBinder keys

Changes the client registration mechanism in ConduitService to use IBinder as the key for tracking registered clients instead of using the AIDL interfaces directly. This fixes an issue where clients were being registered multiple times due to AIDL-generated callback interfaces lacking proper equals/hashCode implementations.

android/app/src/main/java/ca/psiphon/conduit/nativemodule/ConduitService.java

@@ -57,6 +57,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -80,9 +81,6 @@ public class ConduitService extends Service implements PsiphonTunnel.HostService
 
     private final String NOTIFICATION_CHANNEL_ID = "ConduitServiceChannel";
 
-    // Lock to synchronize access to the clients list
-    private final Object clientsLock = new Object();
-
     // Enum to represent the state of the foreground service
     // This tracks the lifecycle of the service in foreground, whether it is
     // running a foreground task (e.g., in-proxy) or transitioning between states.
@@ -96,9 +94,11 @@ private enum ForegroundServiceState {
     // Variable to track the current state of the foreground service
     private final AtomicReference<ForegroundServiceState> foregroundServiceState = new AtomicReference<>(ForegroundServiceState.STOPPED);
 
-    // List to hold the registered clients
-    private final List<IConduitClientCallback> clients = new ArrayList<>();
+    // Map to hold the registered clients
+    private final Map<IBinder, IConduitClientCallback> clients = new ConcurrentHashMap<>();
 
+    // Lock to synchronize access to the clients map
+    private final Object clientsLock = new Object();
 
     // PsiphonTunnel instance
     private final PsiphonTunnel psiphonTunnel = PsiphonTunnel.newPsiphonTunnel(this);
@@ -113,8 +113,12 @@ private enum ForegroundServiceState {
         @Override
         public void registerClient(IConduitClientCallback client) {
             synchronized (clientsLock) {
-                if (client != null && !clients.contains(client)) {
-                    clients.add(client);
+                if (client == null) {
+                    return;
+                }
+                IBinder clientBinder = client.asBinder();
+                if (!clients.containsKey(clientBinder)) {
+                    clients.put(clientBinder, client);
 
                     // Also update the client immediately with the current state and stats
                     // Send state
@@ -750,11 +754,11 @@ private void cancelErrorNotifications() {
 
 
     // Unified method to send updates to all registered clients
-    // This method is synchronized to avoid concurrent modification of the clients list when called from multiple threads
+    // This method is synchronized to avoid concurrent modification of the clients map when called from multiple threads
     private void notifyClients(ClientNotifier notifier) {
         synchronized (clientsLock) {
-            for (Iterator<IConduitClientCallback> iterator = clients.iterator(); iterator.hasNext(); ) {
-                IConduitClientCallback client = iterator.next();
+            for (Iterator<Map.Entry<IBinder, IConduitClientCallback>> iterator = clients.entrySet().iterator(); iterator.hasNext(); ) {
+                IConduitClientCallback client = iterator.next().getValue();
                 try {
                     notifier.notify(client);
                 } catch (RemoteException e) {