ElasticChangeIndex: reuse change index name string

Align such reuse with the current one in the sibling ElasticAccountIndex
and ElasticGroupIndex classes.

Change-Id: I5ccf05b279fab49b16c803f3cf4db6b560fc4983

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java

diff --git a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
index 4f90de5..15185a1 100644
--- a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
+++ b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
@@ -91,9 +91,10 @@
     }
   }
 
-  static final String CHANGES_PREFIX = "changes_";
-  static final String OPEN_CHANGES = "open_changes";
-  static final String CLOSED_CHANGES = "closed_changes";
+  static final String CHANGES = "changes";
+  static final String CHANGES_PREFIX = CHANGES + "_";
+  static final String OPEN_CHANGES = "open_" + CHANGES;
+  static final String CLOSED_CHANGES = "closed_" + CHANGES;
 
   private final ChangeMapping mapping;
   private final Provider<ReviewDb> db;
@@ -134,7 +135,7 @@
     Bulk bulk =
         new Bulk.Builder()
             .defaultIndex(indexName)
-            .defaultType("changes")
+            .defaultType(CHANGES)
             .addAction(insert(insertIndex, cd))
             .addAction(delete(deleteIndex, cd.getId()))
             .refresh(true)