Replace sTGC time-tdo constant with static variable

Similar changes to MR !52717 (merged), because it was found that the shift value of the "place holder" time-TDO conversion wasn't sufficient due to a Gaussian smearing. Also the changes fix the FPE warnings reported in ATR-25352. A proper NSW calibration is under development, and will replace the place holder time-TDO conversion. However, the calibration might not work in RDO transient-persistent converters. So this place holder would be used in MuonRDO/STGC_RawDataCnv_p1.cxx to preserve backward compatibility, though there is not a single realistic case where the _p1 version is needed now or in the future.