Restruct LayerRNDT creation

[etj]

In GeoNode/geonode#7263 a new hook has been added for creating custom data.

Since we may need values extracted from the optional metadata when creating a new Layer, the LayerRNDT instance creation should be implemented using such a hook.

We'll maintan the old logic as well, in order to be able to add RNDT info to existent layer.
The new logic will be used for brand new layers.