ReviewerResource: Don't inherit from ChangeResource

Prefer composition to inheritance. This one was particularly confusing
because of the two different users/controls (caller and reviewer).
Continue the disambiguating started by the previous commit and give
methods slightly more descriptive names.

Change-Id: I3ce101d32cb133698c20869649576808f3240d4b

gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java

@@ -68,7 +68,7 @@ public class ReviewerJson {
     for (ReviewerResource rsrc : rsrcs) {
       ReviewerInfo info = format(new ReviewerInfo(
           rsrc.getReviewerUser().getAccountId()),
-          rsrc.getUserControl());
+          rsrc.getReviewerControl());
       loader.put(info);
       infos.add(info);
     }