YARN-6102. RMActiveService context to be updated with new RMContext on failover. Contributed by Rohith Sharma K S.

(cherry picked from commit a657472b42c58f87fd3165e0a746d83b72182a24)

Conflicts:
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/AdminService.java
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/RMActiveServiceContext.java
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/RMContextImpl.java

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/ResourceManager.java

@@ -117,15 +117,6 @@ import org.apache.hadoop.yarn.webapp.WebApps.Builder;
 import org.apache.hadoop.yarn.webapp.util.WebAppUtils;
 import org.apache.zookeeper.server.auth.DigestAuthenticationProvider;
 
-import java.io.IOException;
-import java.io.InputStream;
-import java.io.PrintStream;
-import java.net.InetSocketAddress;
-import java.nio.charset.Charset;
-import java.security.PrivilegedExceptionAction;
-import java.security.SecureRandom;
-import java.util.ArrayList;
-import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
 import com.google.common.annotations.VisibleForTesting;