[java] Fix the logic of generating TaskID (#2747)

## What do these changes do?
Because the logic of generating `TaskID` in java is different from python's, there are many tests fail when we change the `Ray Core` code.
In this change,  I rewrote the logic of generating `TaskID` in java which is the same as the python's.

In java, we call the native method `_generateTaskId()` to generate a `TaskID` which is also used in python. We change `computePutId()`'s logic too.

## Related issue number
[#2608](https://github.com/ray-project/ray/issues/2608)

java/test/src/main/java/org/ray/api/test/UniqueIdTest.java

@@ -0,0 +1,84 @@
+package org.ray.api.test;
+
+import java.nio.ByteBuffer;
+import java.util.Arrays;
+import javax.xml.bind.DatatypeConverter;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.ray.api.UniqueID;
+import org.ray.core.UniqueIdHelper;
+
+@RunWith(MyRunner.class)
+public class UniqueIdTest {
+
+  @Test
+  public void testConstructUniqueId() {
+    // Test `fromHexString()`
+    UniqueID id1 = UniqueID.fromHexString("00000000123456789ABCDEF123456789ABCDEF00");
+    Assert.assertEquals("00000000123456789ABCDEF123456789ABCDEF00", id1.toString());
+    Assert.assertFalse(id1.isNil());
+
+    try {
+      UniqueID id2 = UniqueID.fromHexString("000000123456789ABCDEF123456789ABCDEF00");
+      // This shouldn't be happened.
+      Assert.assertTrue(false);
+    } catch (IllegalArgumentException e) {
+      Assert.assertTrue(true);
+    }
+
+    try {
+      UniqueID id3 = UniqueID.fromHexString("GGGGGGGGGGGGG");
+      // This shouldn't be happened.
+      Assert.assertTrue(false);
+    } catch (IllegalArgumentException e) {
+      Assert.assertTrue(true);
+    }
+
+    // Test `fromByteBuffer()`
+    byte[] bytes = DatatypeConverter.parseHexBinary("0123456789ABCDEF0123456789ABCDEF01234567");
+    ByteBuffer byteBuffer = ByteBuffer.wrap(bytes, 0, 20);
+    UniqueID id4 = UniqueID.fromByteBuffer(byteBuffer);
+    Assert.assertTrue(Arrays.equals(bytes, id4.getBytes()));
+    Assert.assertEquals("0123456789ABCDEF0123456789ABCDEF01234567", id4.toString());
+
+
+    // Test `genNil()`
+    UniqueID id6 = UniqueID.genNil();
+    Assert.assertEquals("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF", id6.toString());
+    Assert.assertTrue(id6.isNil());
+  }
+
+  @Test
+  public void testComputeReturnId() {
+    // Mock a taskId, and the lowest 4 bytes should be 0.
+    UniqueID taskId = UniqueID.fromHexString("00000000123456789ABCDEF123456789ABCDEF00");
+
+    UniqueID returnId = UniqueIdHelper.computeReturnId(taskId, 1);
+    Assert.assertEquals("01000000123456789ABCDEF123456789ABCDEF00", returnId.toString());
+
+    returnId = UniqueIdHelper.computeReturnId(taskId, 0x01020304);
+    Assert.assertEquals("04030201123456789ABCDEF123456789ABCDEF00", returnId.toString());
+  }
+
+  @Test
+  public void testComputeTaskId() {
+    UniqueID objId = UniqueID.fromHexString("34421980123456789ABCDEF123456789ABCDEF00");
+    UniqueID taskId = UniqueIdHelper.computeTaskId(objId);
+
+    Assert.assertEquals("00000000123456789ABCDEF123456789ABCDEF00", taskId.toString());
+  }
+
+  @Test
+  public void testComputePutId() {
+    // Mock a taskId, the lowest 4 bytes should be 0.
+    UniqueID taskId = UniqueID.fromHexString("00000000123456789ABCDEF123456789ABCDEF00");
+
+    UniqueID putId = UniqueIdHelper.computePutId(taskId, 1);
+    Assert.assertEquals("FFFFFFFF123456789ABCDEF123456789ABCDEF00", putId.toString());
+
+    putId = UniqueIdHelper.computePutId(taskId, 0x01020304);
+    Assert.assertEquals("FCFCFDFE123456789ABCDEF123456789ABCDEF00", putId.toString());
+  }
+
+}