Accommodate multiple collections with ReadHandle

When working with two collections -- one for hard-scatter and another for pile-up -- we need to look at different StoreGates. Previously, when the AthAlgTool::EvtStore property was overridden in JOs, this would also effect the global SG key.

This patch ensures that the TruthEvent is retrieved from the default StoreGate instance -- i.e. that named StoreGateSvc -- rather than the OriginalEvent_SG when using multiple collections.