feat(screenshare): support remote wireless screensharing (#3809)

* feat(screenshare): support remote wireless screensharing

- Pass events to the ProxyConnectionService so it can
  handle establishing a peer connection so a remote
  participant, not in the conference, can send a
  video stream to the local participant to use as a
  local desktop stream.
- Modify the existing start screensharing flow to accept
  a desktop stream instead of always trying to create one.

* adjust ProxyConnectionService for lib review changes

modules/API/API.js

@@ -60,6 +60,9 @@ function initCommands() {
             sendAnalytics(createApiEvent('display.name.changed'));
             APP.conference.changeLocalDisplayName(displayName);
         },
+        'proxy-connection-event': event => {
+            APP.conference.onProxyConnectionEvent(event);
+        },
         'submit-feedback': feedback => {
             sendAnalytics(createApiEvent('submit.feedback'));
             APP.conference.submitFeedback(feedback.score, feedback.message);
@@ -260,6 +263,20 @@ class API {
         });
     }
 
+    /**
+     * Notifies the external application (spot) that the local jitsi-participant
+     * has a status update.
+     *
+     * @param {Object} event - The message to pass onto spot.
+     * @returns {void}
+     */
+    sendProxyConnectionEvent(event: Object) {
+        this._sendEvent({
+            name: 'proxy-connection-event',
+            ...event
+        });
+    }
+
     /**
      * Sends event to the external application.
      *