Update comment on job ID.

Update the comment on the job ID to indicate that the meaning is
different on U+ devices.

Bug: 138239687
Test: statsd_testdrive 8
Change-Id: Ib86f55705cae3f36702a7ca3d05877d6df499100
diff --git a/stats/atoms.proto b/stats/atoms.proto
index 08ae8ff..3ca64fb 100644
--- a/stats/atoms.proto
+++ b/stats/atoms.proto
@@ -2183,8 +2183,9 @@
     }
     optional Bucket standby_bucket = 5 [default = UNKNOWN];
 
-    // The job id (as assigned by the app).
-    optional int32 job_id = 6;
+    // On T- builds, this is the job id (as assigned by the app).
+    // On U+ builds, this is generated by the system, factoring in the job's namespace.
+    optional int64 job_id = 6;
 
     // One flag for each of the API constraints defined by JobScheduler. Does not include implicit
     // constraints as they are always assumed to be set.