screenshare: fix issue where screenshare didnt close

When joining a callSwarm as a participant, your mediaList would
contain 3 medias if a stream and video were active. The logic
behind this function didn't take that into account. In the future
it would be good to re-write this for arbitrary amounts of
medias.

Change-Id: If128650c535ae8ba9b02ec4738e8ef103f4994df

src/libclient/api/call.h

@@ -172,7 +172,7 @@ struct Info
                         callInfo["is_sharing"] = false;
                     } else {
                         callInfo["is_sharing"] = true;
-                        callInfo["preview_id"] = media[MediaAttributeKey::SOURCE];
+                        previewId = media[MediaAttributeKey::SOURCE];
                     }
                 }
                 if (media[MediaAttributeKey::ENABLED] == TRUE_STR