Use the hst manager jcr session when building list of channels

The channel manager impl uses a session provided by a threadlocal to read the channel information. However, this session might not always have the correct authorization and this makes the retrieved information dependent on the client.

Using the same jcr session that's used by the hst manager guarantees that the information is in sync and always the same, irrespective of the client of the channel manager service. The client credentials should only be used when modifying the channel configuration stored in the repository.