Make UI.pushConnection transient to prevent null resource after deserialization (#11809)

* PushConnection is not Serializable anymore
* AtmospherePushConnection fields are not transient
* UI.setSession calls setPushConnection(null) instead of pushConnection.disconnect()
* pushConnection.disconnect() asserts isConnected()
* If UI has a push connection, it should now always have isConnected() == true

Change-Id: I3c2e877b8e723b7cc2993cacd620920aecdef5fb