LArConditionsCommon: Fixes for LAr conditions setup.

LAr conditions setup was failing for run 1, where we didn't have
flat conditions.  Try to make work now for all cases.

There are still some components that access conditions directly,
not using a conditions handle.  For those cases, we still need
to keep LArFlatConditionsSvc alive.  Once those are gone,
we can remove it.



Former-commit-id: 2bbe8118