fix(conference): Don't use this._room.

this._room should be used only by jitsi-meet-torture and for test purposes. Also this._room is assigned later than room. This may cause some issues, for example conference.getMyUserId() may return undefined while the user id is already available beacuse this._room hasn't been assigned yet.

conference.js

      * Returns the connection times stored in the library.
      */
     getConnectionTimes() {
-        return this._room.getConnectionTimes();
+        return room.getConnectionTimes();
     },
 
     // used by torture currently
     isJoined() {
-        return this._room
-            && this._room.isJoined();
+        return room && room.isJoined();
     },
     getConnectionState() {
-        return this._room
-            && this._room.getConnectionState();
+        return room && room.getConnectionState();
     },
 
     /**
      * P2P connection
      */
     getP2PConnectionState() {
-        return this._room
-            && this._room.getP2PConnectionState();
+        return room && room.getP2PConnectionState();
     },
 
     /**
      */
     _startP2P() {
         try {
-            this._room && this._room.startP2PSession();
+            room && room.startP2PSession();
         } catch (error) {
             logger.error('Start P2P failed', error);
             throw error;
      */
     _stopP2P() {
         try {
-            this._room && this._room.stopP2PSession();
+            room && room.stopP2PSession();
         } catch (error) {
             logger.error('Stop P2P failed', error);
             throw error;
      * false otherwise.
      */
     isConnectionInterrupted() {
-        return this._room.isConnectionInterrupted();
+        return room.isConnectionInterrupted();
     },
 
     /**
     },
 
     getMyUserId() {
-        return this._room && this._room.myUserId();
+        return room && room.myUserId();
     },
 
     /**
      * least one track.
      */
     getNumberOfParticipantsWithTracks() {
-        return this._room.getParticipants()
+        return room.getParticipants()
             .filter(p => p.getTracks().length > 0)
             .length;
     },
 
         if (config.requireDisplayName
                 && !APP.conference.getLocalDisplayName()
-                && !this._room.isHidden()) {
+                && !room.isHidden()) {
             APP.UI.promptDisplayName();
         }