Add back the onCreateTime check in the background runnable

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java

@@ -593,7 +593,9 @@ public StartFromBackgroundRunnable(final AppStartTrace trace) {
 
     @Override
     public void run() {
-      trace.mainThreadRunnableTime = new Timer();
+      if (trace.onCreateTime == null) {
+        trace.mainThreadRunnableTime = new Timer();
+      }
     }
   }