StiIstDetectorBuilder: A couple minor edits for consistency across StXXXDetectorBuilders:

- Switched to same names as in StiPxlDetectorBuilder
- Switched to natural numbering scheme as in StiPxlDetectorBuilder
Use the center sensor (or the one close to the center) to build the unified sensor layer in Sti - inspired by Hao Qiu
Split PXL sensitive layers in two halves. The change should help to avoid track backward steps in Sti due to ill ordered volumes in r and phi - inspired by Hao Qiu