Simplify contact and organizer ID representations

Simplify contact and organizer IDs to simply contain a manager URI
that identifies the owner of the identified object, and an engine-
specific string that identifies the object to that manager.

Change-Id: Ic14c419df264e91c61e0f3d474d846289e1aaa82
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>