[sim] Use plant output to retrieve simulated position (#3043)

Using the plant output means that measurement noise can be incorporated.
SingleJointedArmSim (in C++ and Java) and ElevatorSim (in C++) used the
state instead of the measurement.

Closes #3042

wpilibj/src/main/java/edu/wpi/first/wpilibj/simulation/SingleJointedArmSim.java

@@ -199,7 +199,7 @@ public class SingleJointedArmSim extends LinearSystemSim<N2, N1, N1> {
    * @return The current arm angle.
    */
   public double getAngleRads() {
-    return m_x.get(0, 0);
+    return m_y.get(0, 0);
   }
 
   /**