AthenaServices: Define timestamp in dummy event context.

In AthenaEventLoopMgr when there is no event selector defined, we need to create a dummy event context. Previously, the EventID in this dummy context defined run/event/lumi numbers, but not a timestamp. This was observed to cause problems in MuonAlignmentCondAlg, which accesses conditions during initialize(), in tests that don't define an input file. Define a dummy timestamp as well to prevent this.