Include the session id in TextClassifierApiUsageReported

Just like the other TC events, log also the session id. This allows us
to relate the collected latency to things like model version.

MDD: eldar/assessments/913360069/revisions/2
PDD: eldar/assessments/576513345/drafts/528750673

Bug: 169042373
Test: m

Merged-In: I86580e87cdd57942db23cd10509db02be6eea6aa

Change-Id: I86580e87cdd57942db23cd10509db02be6eea6aa
diff --git a/stats/atoms.proto b/stats/atoms.proto
index 18a0ab0..bd61f3c 100644
--- a/stats/atoms.proto
+++ b/stats/atoms.proto
@@ -11272,4 +11272,5 @@
     }
     optional ResultType result_type = 2;
     optional int64 latency_millis = 3;
+    optional string session_id = 4;
 }