Fix layerID method in FT DD4HEP by using layerIdx
See Rec#382 (@jonrob @gligorov )
Should fix properly the "layer starting from 4" issue.
For note, an even more proper fix would overhaul how IDs are inconsistent between DetDesc and DD4HEP, and how DetDesc IDs are inconsistent between themselves (some are indices, some are local, some are global, from what I see). I do not think this is warranted for now.
Should be merged with Rec!3055 (merged)