android: fix synchronized access to listeners set

Fixes this issue:

~~~
    java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextNode(HashMap.java:1441)
        at java.util.HashMap$KeyIterator.next(HashMap.java:1465)
        at org.jitsi.meet.sdk.OngoingConferenceTracker.updateListeners(OngoingConferenceTracker.java:89)
        at org.jitsi.meet.sdk.OngoingConferenceTracker.onExternalAPIEvent(OngoingConferenceTracker.java:74)
        at org.jitsi.meet.sdk.ExternalAPIModule.sendEvent(ExternalAPIModule.java:71)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.facebook.react.bridge.JavaMethodWrapper.invoke(JavaMethodWrapper.java:372)
        at com.facebook.react.bridge.JavaModuleWrapper.invoke(JavaModuleWrapper.java:158)
        at com.facebook.react.bridge.queue.NativeRunnable.run(Native Method)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:29)
        at android.os.Looper.loop(Looper.java:214)
        at com.facebook.react.bridge.queue.MessageQueueThreadImpl$4.run(MessageQueueThreadImpl.java:232)
        at java.lang.Thread.run(Thread.java:764)
~~~

android/sdk/src/main/java/org/jitsi/meet/sdk/OngoingConferenceTracker.java

@@ -86,8 +86,10 @@ class OngoingConferenceTracker {
     }
 
     private void updateListeners() {
-        for (OngoingConferenceListener listener : listeners) {
-            listener.onCurrentConferenceChanged(currentConference);
+        synchronized (listeners) {
+            for (OngoingConferenceListener listener : listeners) {
+                listener.onCurrentConferenceChanged(currentConference);
+            }
         }
     }