fix(e2ee) fix race condition when restarting media sessions

Make sure the P2P tracks are not added to the JVB session when we are restarting
the media sessions, since the PC has not been created with the encoded streams
constraint yet.

JitsiConference.js

         // Let the RTC service do any cleanups
         this.rtc.onCallEnded();
     } else if (jingleSession === this.p2pJingleSession) {
+        const stopOptions = {};
+
         // It's the responder who decides to enforce JVB mode, so that both
         // initiator and responder are aware if it was intentional.
         if (reasonCondition === 'decline' && reasonText === 'force JVB121') {
             // terminates the session, before we get the event on our side.
             // But we are able to parse the reason and mark it here.
             Statistics.analytics.addPermanentProperties({ p2pFailed: true });
+        } else if (reasonCondition === 'success' && reasonText === 'restart') {
+            // When we are restarting media sessions we don't want to switch the tracks
+            // to the JVB just yet.
+            stopOptions.requestRestart = true;
         }
-        this._stopP2PSession();
+        this._stopP2PSession(stopOptions);
     } else {
         logger.error(
             'Received onCallEnded for invalid session',
                     }));
 
         }
-        this._stopP2PSession('connectivity-error', 'ICE FAILED');
+        this._stopP2PSession({
+            reason: 'connectivity-error',
+            reasonDescription: 'ICE FAILED'
+        });
     } else if (session && this.jvbJingleSession === session) {
         this._delayedIceFailed = new IceFailedHandling(this);
         this._delayedIceFailed.start(session);
         && Math.random() < forceJVB121Ratio) {
         logger.info(`Forcing JVB 121 mode (ratio=${forceJVB121Ratio})...`);
         Statistics.analytics.addPermanentProperties({ forceJvb121: true });
-        this._stopP2PSession('decline', 'force JVB121');
+        this._stopP2PSession({
+            reason: 'decline',
+            reasonDescription: 'force JVB121'
+        });
 
         done = true;
     }
 
 /**
  * Stops the current P2P session.
- * @param {string} [reason="success"] one of the Jingle "reason" element
+ * @param {Object} options - Options for stopping P2P.
+ * @param {string} options.reason - One of the Jingle "reason" element
  * names as defined by https://xmpp.org/extensions/xep-0166.html#def-reason
- * @param {string} [reasonDescription="Turing off P2P session"] text
+ * @param {string} options.reasonDescription - Text
  * description that will be included in the session terminate message
+ * @param {boolean} requestRestart - Whether this is due to a session restart, in which case
+ * media will not be resumed on the JVB.
  * @private
  */
-JitsiConference.prototype._stopP2PSession = function(
-        reason,
-        reasonDescription) {
+JitsiConference.prototype._stopP2PSession = function(options = {}) {
+    const {
+        reason = 'success',
+        reasonDescription = 'Turning off P2P session',
+        requestRestart = false
+    } = options;
+
     if (!this.p2pJingleSession) {
         logger.error('No P2P session to be stopped!');
 
 
     // Swap remote tracks, but only if the P2P has been fully established
     if (wasP2PEstablished) {
-        if (this.jvbJingleSession) {
+        if (this.jvbJingleSession && !requestRestart) {
             this._resumeMediaTransferForJvbConnection();
         }
 
                         + ' P2P Jingle session', error);
             }
         }, {
-            reason: reason ? reason : 'success',
-            reasonDescription: reasonDescription
-                ? reasonDescription : 'Turing off P2P session',
+            reason,
+            reasonDescription,
             sendSessionTerminate: this.room
                 && this.getParticipantById(
                     Strophe.getResourceFromJid(this.p2pJingleSession.remoteJid))
 
     if (wasP2PEstablished) {
         // Add back remote JVB tracks
-        if (this.jvbJingleSession) {
+        if (this.jvbJingleSession && !requestRestart) {
             this._addRemoteJVBTracks();
         } else {
             logger.info('Not adding remote JVB tracks - no session yet');
 };
 
 /**
- * Manually stops the current P2P session (should be used only in the tests)
+ * Manually stops the current P2P session (should be used only in the tests).
  */
-JitsiConference.prototype.stopP2PSession = function() {
-    this._stopP2PSession();
+JitsiConference.prototype.stopP2PSession = function(options) {
+    this._stopP2PSession(options);
 };
 
 /**
  */
 JitsiConference.prototype._restartMediaSessions = function() {
     if (this.p2pJingleSession) {
-        this.stopP2PSession();
+        this._stopP2PSession({
+            reasonDescription: 'restart',
+            requestRestart: true
+        });
     }
 
     if (this.jvbJingleSession) {