fix(lastN): Return the correct lastN value for the conference.

JitsiConference.js

  * @returns {number}
  */
 JitsiConference.prototype.getLastN = function() {
-    return this.rtc.getLastN();
+    return this.receiveVideoController.getLastN();
 };
 
 /**
 
         // Sync up video transfer active in case p2pJingleSession not existed
         // when the lastN value was being adjusted.
-        const isVideoActive = this.rtc.getLastN() !== 0;
+        const isVideoActive = this.getLastN() !== 0;
 
         this.p2pJingleSession
             .setMediaTransferActive(true, isVideoActive)

modules/qualitycontrol/ReceiveVideoController.js

         this._maxFrameHeight && mediaSession.setReceiverVideoConstraint(this._maxFrameHeight);
     }
 
+    /**
+     * Returns the lastN value for the conference.
+     *
+     * @returns {number}
+     */
+    getLastN() {
+        return this._lastN;
+    }
+
     /**
      * Elects the participants with the given ids to be the selected participants in order to always receive video
      * for this participant (even when last n is enabled).