rn: don't start native call integration multiple times

When you join a conference that needs an authenticated moderator, as a guest, Jitsi Meet will continuously try and connect to the meeting every 5 seconds. Avoid starting the native call integration more than once.

Fixes: https://github.com/jitsi/jitsi-meet/issues/6260

react/features/mobile/call-integration/middleware.js

     const handle = callHandle || url.toString();
     const hasVideo = !isVideoMutedByAudioOnly(state);
 
+    // If we already have a callUUID set, don't start a new call.
+    if (conference.callUUID) {
+        return result;
+    }
+
     // When assigning the call UUID, do so in upper case, since iOS will return
     // it upper cased.
     conference.callUUID = (callUUID || uuid.v4()).toUpperCase();