YARN-4205. Addendum patch to fix branch-2.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/rmapp/monitor/RMAppLifetimeMonitor.java

@@ -100,7 +100,10 @@ public class RMAppLifetimeMonitor
       ApplicationTimeoutType timeoutType, long monitorStartTime, long timeout) {
     RMAppToMonitor appToMonitor = new RMAppToMonitor(appId, timeoutType);
     register(appToMonitor, monitorStartTime);
-    monitoredApps.putIfAbsent(appToMonitor, timeout);
+    Long oldTimeout = monitoredApps.get(appToMonitor);
+    if (oldTimeout == null) {
+      monitoredApps.put(appToMonitor, timeout);
+    }
   }
 
   @Override