[hal] Add high level way of getting systemserver NT instance (#7683)

hal/src/main/native/cpp/jni/SystemServerJNI.cpp

@@ -0,0 +1,24 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <jni.h>
+
+#include "edu_wpi_first_hal_SystemServerJNI.h"
+#include "hal/SystemServer.h"
+
+extern "C" {
+
+/*
+ * Class:     edu_wpi_first_hal_SystemServerJNI
+ * Method:    getSystemServerHandle
+ * Signature: ()I
+ */
+JNIEXPORT jint JNICALL
+Java_edu_wpi_first_hal_SystemServerJNI_getSystemServerHandle
+  (JNIEnv*, jclass)
+{
+  return HAL_GetSystemServerHandle();
+}
+
+}  // extern "C"