Cache deserialized manager URIs to promote data sharing

When deserializing ID objects, construct the resulting objects from
cached instances of the manager URI string so that the resulting
ID objects can share a string data representation.

Change-Id: If3091bfc42e61938a823a9d2bfc68d0dafec0977
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>