Add ability to reconnect to the current channel

[JordanMilne]

This'd make transitioning between WiFi and 4G when using Plumble less painful.

Could probably do something like

            try {
                IChannel currChannel = mBinder.getSessionChannel();
                if (currChannel != null)
                    mReconnectChannel = currChannel.getId();
            } catch (RemoteException e) {}}

in setReconnecting() and then rejoin that channel in connect(), but I'm not that clear on how state should be managed between connection attempts, so I haven't been able to get it to work.

[acomminos]

Registered users should be automatically moved into their last channel by the server; is there any particularly good reason to support this if a user is not registered? I would prefer to minimize our local retention of server state.