Disable Atmosphere online/offline handling (#19391)

Atmosphere will disconnect the push connection when going offline and reconnect it
when going online again. There are no events sent when this happens, which means the 
reconnect logic cannot take this into account. 

Having it enabled leads to something like:

1. Browser goes offline, push is disconnected
2. Framework sees it as a normal close and does not show the reconnect dialog
3. The user does something, the framework tries to send a websocket request
4. A new Atmosphere request is created and enters a try-to-reconnect loop as expected
5. The browser goes online, and the online event re-initializes the old connection 
although a try-to-reconnect loop is already running

Change-Id: I5dcef768783142e2c7b4e7c821353addb0ec3d28

client/src/com/vaadin/client/communication/AtmospherePushConnection.java

@@ -520,6 +520,7 @@ public class AtmospherePushConnection implements PushConnection {
             maxReconnectOnClose: 10000000,
             trackMessageLength: true,
             enableProtocol: true,
+            handleOnlineOffline: false,
             messageDelimiter: String.fromCharCode(@com.vaadin.shared.communication.PushConstants::MESSAGE_DELIMITER)
         };
     }-*/;