[Java] Fix some potential bugs about Ray.shutdown() (#5693)

java/runtime/src/main/java/org/ray/runtime/RayDevRuntime.java

@@ -33,6 +33,10 @@ public class RayDevRuntime extends AbstractRayRuntime {
 
   @Override
   public void shutdown() {
+    if (taskSubmitter != null) {
+      ((LocalModeTaskSubmitter) taskSubmitter).shutdown();
+      taskSubmitter = null;
+    }
     taskExecutor = null;
   }
 

java/runtime/src/main/java/org/ray/runtime/RayNativeRuntime.java

@@ -133,6 +133,7 @@ public final class RayNativeRuntime extends AbstractRayRuntime {
     }
     if (null != manager) {
       manager.cleanup();
+      manager = null;
     }
   }
 

java/runtime/src/main/java/org/ray/runtime/task/LocalModeTaskSubmitter.java

@@ -210,6 +210,10 @@ public class LocalModeTaskSubmitter implements TaskSubmitter {
     }
   }
 
+  public void shutdown() {
+    exec.shutdown();
+  }
+
   public static ActorId getActorId(TaskSpec taskSpec) {
     ByteString actorId = null;
     if (taskSpec.getType() == TaskType.ACTOR_CREATION_TASK) {