[Java] Fix out-dated signatures of JNI methods (#2756)

1) Renamed the native JNI methods and some parameters of JNI methods. 
2) Fixed native JNI methods' signatures by `javah` tool.
3) Removed some useless native methods.

java/runtime-common/src/main/java/org/ray/spi/LocalSchedulerLink.java

@@ -11,7 +11,7 @@ public interface LocalSchedulerLink {
 
   void submitTask(TaskSpec task);
 
-  TaskSpec getTaskTodo();
+  TaskSpec getTask();
 
   void markTaskPutDependency(UniqueID taskId, UniqueID objectId);
 

java/runtime-common/src/main/java/org/ray/spi/LocalSchedulerProxy.java

@@ -80,7 +80,7 @@ public class LocalSchedulerProxy {
   }
 
   public TaskSpec getTask() {
-    TaskSpec ts = scheduler.getTaskTodo();
+    TaskSpec ts = scheduler.getTask();
     RayLog.core.info("Task " + ts.taskId.toString() + " received");
     return ts;
   }