Fix many errorprone warnings (#1247)

This fixes two real bugs:
- TimedRobot had a m_period that was hiding the IterativeRobotBase m_period
  and was not getting initialized.
- PDPSim was swapping two parameters to getCurrent()

wpilibj/src/main/java/edu/wpi/first/wpilibj/DriverStation.java

@@ -34,12 +34,12 @@ public class DriverStation {
    */
   public static final int kJoystickPorts = 6;
 
-  private class HALJoystickButtons {
+  private static class HALJoystickButtons {
     public int m_buttons;
     public byte m_count;
   }
 
-  private class HALJoystickAxes {
+  private static class HALJoystickAxes {
     public float[] m_axes;
     public short m_count;
 
@@ -48,7 +48,7 @@ public class DriverStation {
     }
   }
 
-  private class HALJoystickPOVs {
+  private static class HALJoystickPOVs {
     public short[] m_povs;
     public short m_count;