Convert PatchSet.Id to AutoValue

See I6982fb24 for context.

Change-Id: I681421e2e3415125dc4db44c7f3bf32d37920166
diff --git a/proto/entities.proto b/proto/entities.proto
index 0855265..f263a2b 100644
--- a/proto/entities.proto
+++ b/proto/entities.proto
@@ -84,7 +84,7 @@
 // Next ID: 3
 message PatchSet_Id {
   required Change_Id change_id = 1;
-  required int32 patch_set_id = 2;
+  required int32 id = 2;
 }
 
 // Serialized form of com.google.gerrit.reviewdb.client.PatchSet.