[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-dev/src/main/java/org/ray/spi/impl/MockLocalScheduler.java

@@ -65,7 +65,7 @@ public class MockLocalScheduler implements LocalSchedulerLink {
   }
 
   @Override
-  public TaskSpec getTaskTodo() {
+  public TaskSpec getTask() {
     throw new RuntimeException("invalid execution flow here");
   }