ref(mobile/external-api): skip event if conference exists

The change to mobile/external-api is required to not emit
CONFERENCE_FAILED for CONNECTION_FAILED if the conference has been
started, because base/conference state will still hold conference
instances which are to be ended by other means and result in the
appropriate event (which will adjust the base/conference state).

react/features/mobile/external-api/middleware.js

 
 /**
  * Sends {@link CONFERENCE_FAILED} event when the {@link CONNECTION_FAILED}
- * occurs. Otherwise the external API will not emit such event, because at this
- * point conference has not been created yet and the base/conference feature
- * will not emit it.
+ * occurs. It should be done only if the connection fails before the conference
+ * instance is created. Otherwise the eventual failure event is supposed to be
+ * emitted by the base/conference feature.
  *
  * @param {Store} store - The redux store.
  * @param {Action} action - The redux action.
  */
 function _sendConferenceFailedOnConnectionError(store, action) {
     const { locationURL } = store.getState()['features/base/connection'];
+    const { connection } = action;
 
-    locationURL && _sendEvent(
+    locationURL
+        && forEachConference(
+            store,
+
+            // If there's any conference in the  base/conference state then the
+            // base/conference feature is supposed to emit a failure.
+            conference => conference.getConnection() !== connection)
+        && _sendEvent(
         store,
         _getSymbolDescription(CONFERENCE_FAILED),
         /* data */ {