Add SSL Proxy support for Glass Mirror API Java Sample

MirrorClient.java

  /**
   * Subscribes to notifications on the user's timeline.
   */
  public static Subscription insertSubscription(Credential credential, String callbackUrl,
    ...
    // Rewrite "appspot.com" to "Appspot.com" as a workaround for
    // http://b/6909300.
    callbackUrl = callbackUrl.replace("appspot.com", "Appspot.com");
    // add ssl proxy
    callbackUrl = "https://mirrornotifications.appspot.com/forward?url=" + callbackUrl;
    
    Subscription subscription = new Subscription();
    ...
  }