YARN-6000. Make AllocationFileLoaderService.Listener public. (Tao Jie via kasha)

(cherry picked from commit 4d3f73acc0a5cabc748132889dbe670bea178a3f)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/AllocationFileLoaderService.java

@@ -639,7 +639,7 @@ public class AllocationFileLoaderService extends AbstractService {
     return defaultPermissions;
   }
 
-  interface Listener {
+  public interface Listener {
     void onReload(AllocationConfiguration info) throws IOException;
   }
 }