PublishingChannelInitializer.allChannels is final

Add final keyword as appropriate.

Change-Id: Idfbadf4f1cc6f296fddcaaf49a2fb18775d63671
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>