[sessions] Make session more robust.

This commit fixes issue with hardcoded userid. It also introduces some
changes that are expected by some middleware components.

Signed-off-by: Marko Saukko <marko.saukko@jollamobile.com>
[userids] Fix user ids to match the new ids in Mer.

Signed-off-by: Marko Saukko <marko.saukko@jollamobile.com>