SimplePush: use pushEndpoint URL as device token

For SimplePush we now use the pushEndpoint URL as device-token instead of having it its own field. We also no longer store channelID, was device-token in the past

Done for https://issues.jboss.org/browse/AGPUSH-532


__**NOTE**__: Please do not merge this, until we have a PR for [AGPUSH-802](https://issues.jboss.org/browse/AGPUSH-802)