[sim] Move Sim classes from HAL to wpilibc/j (#2549)

Also move some things in HAL for consistency.

WAS:
C++:
- C APIs: #include "mockdata/AccelerometerData.h"
- User side class: #include "simulation/AccelerometerSim.h"
Java:
- JNI APIs: hal.sim.mockdata.AccelerometerData (and a few classes in hal.sim)
- User side classes: hal.sim.AccelerometerSim

IS:
C++:
- C APIs: #include "hal/simulation/AccelerometerData.h"
- C++ class: #include "frc/simulation/AccelerometerSim.h"
Java:
- JNI APIs: hal.simulation.AccelerometerData
- User side class: wpilibj.simulation.AccelerometerSim

wpilibc/src/test/native/cpp/simulation/AccelerometerSimTest.cpp

@@ -0,0 +1,40 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "frc/simulation/AccelerometerSim.h"  // NOLINT(build/include_order)
+
+#include <hal/Accelerometer.h>
+#include <hal/HAL.h>
+
+#include "gtest/gtest.h"
+
+using namespace frc::sim;
+
+TEST(AcclerometerSimTests, TestActiveCallback) {
+  HAL_Initialize(500, 0);
+
+  AccelerometerSim sim{0};
+
+  sim.ResetData();
+
+  bool wasTriggered = false;
+  bool lastValue = false;
+
+  auto cb = sim.RegisterActiveCallback(
+      [&](wpi::StringRef name, const HAL_Value* value) {
+        wasTriggered = true;
+        lastValue = value->data.v_boolean;
+      },
+      false);
+
+  EXPECT_FALSE(wasTriggered);
+
+  HAL_SetAccelerometerActive(true);
+
+  EXPECT_TRUE(wasTriggered);
+  EXPECT_TRUE(lastValue);
+}