Convert AccountGroup.UUID to AutoValue

Adapt GroupReference to avoid an AccountGroup.UUID with a null value.

See I6982fb24 for context.

Change-Id: I26c460304eed8ffa77ba65b712ed4ffb73974286
diff --git a/java/com/google/gerrit/elasticsearch/ElasticGroupIndex.java b/java/com/google/gerrit/elasticsearch/ElasticGroupIndex.java
index ecda1ee..471bc4e 100644
--- a/java/com/google/gerrit/elasticsearch/ElasticGroupIndex.java
+++ b/java/com/google/gerrit/elasticsearch/ElasticGroupIndex.java
@@ -117,8 +117,7 @@
     }
 
     AccountGroup.UUID uuid =
-        new AccountGroup.UUID(
-            source.getAsJsonObject().get(GroupField.UUID.getName()).getAsString());
+        AccountGroup.uuid(source.getAsJsonObject().get(GroupField.UUID.getName()).getAsString());
     // Use the GroupCache rather than depending on any stored fields in the
     // document (of which there shouldn't be any).
     return groupCache.get().get(uuid).orElse(null);