[CobaltService] Fix EventAggregatorManager constructor

This was discovered while trying to switch Fuchsia to use CobaltService.

Change-Id: I793b790969e3f0c4b679b0cddb4707f89c8e4da4
diff --git a/src/local_aggregation/event_aggregator_mgr.cc b/src/local_aggregation/event_aggregator_mgr.cc
index 0dc0810..6310e5d 100644
--- a/src/local_aggregation/event_aggregator_mgr.cc
+++ b/src/local_aggregation/event_aggregator_mgr.cc
@@ -174,6 +174,7 @@
       owned_obs_history_proto_store_.get(), backfill_days_);
 
   event_aggregator_ = std::make_unique<EventAggregator>(aggregate_store_.get());
+  steady_clock_ = std::make_unique<SteadyClock>();
 }
 
 }  // namespace cobalt::local_aggregation