Implement /groups/?user=* in terms of /accounts/*/groups

Reuse the same view implementation to select groups of
the target user, but formated as a GroupMap.

Change-Id: I9ceba441f59119cf61becc36acbfa3166da85444

gerrit-server/src/main/java/com/google/gerrit/server/account/GetGroups.java

@@ -25,7 +25,7 @@ import com.google.inject.Inject;
 
 import java.util.List;
 
-class GetGroups implements RestReadView<AccountResource> {
+public class GetGroups implements RestReadView<AccountResource> {
   private final GroupControl.Factory groupControlFactory;
 
   @Inject