Allow first uid in attribution chain as primary field

Test: bit statsd_test:*
Bug: 142108433
Change-Id: I6a2f8e6198765d4205dac95e5f54666496b78808
diff --git a/tools/stats_log_api_gen/Collation.h b/tools/stats_log_api_gen/Collation.h
index 3efdd52..87d4d5d 100644
--- a/tools/stats_log_api_gen/Collation.h
+++ b/tools/stats_log_api_gen/Collation.h
@@ -36,6 +36,8 @@
 
 const int PULL_ATOM_START_ID = 10000;
 
+const int FIRST_UID_IN_CHAIN_ID = 0;
+
 /**
  * The types for atom parameters.
  */