Added field checking on stopID

Moved around isValid checking: Now just create an arrival around a stopid,
and use the arrival's isValid instead of LaMetroUtil. LaMetroUtil isValid
now only checks raw strings, as passed from the arrivals.

This change results in the isValid() check in TripPopulator now correctly
ignoring input. The previous crashes were actually in LaMetroUtil.

This has solved StopID-related input crashing.