[wpilib] RobotController: Add isSystemTimeValid() (#5696)

This returns true when the system date/time (wall clock) is valid.

hal/src/main/native/sim/HAL.cpp

@@ -336,6 +336,10 @@ HAL_Bool HAL_GetRSLState(int32_t* status) {
   return false;
 }
 
+HAL_Bool HAL_GetSystemTimeValid(int32_t* status) {
+  return true;
+}
+
 HAL_Bool HAL_Initialize(int32_t timeout, int32_t mode) {
   static std::atomic_bool initialized{false};
   static wpi::mutex initializeMutex;