Disable frivolous PMD warnings and enable PMD in ntcore (#3419)

Some valid warnings like throwing NullPointerException or using a for
loop instead of System.arraycopy() were fixed.

Abstract classes marked with PMD.AbstractClassWithoutAbstractMethod were
made concrete because they already had protected constructors.

Fixes #1697.

wpilibj/src/main/java/edu/wpi/first/wpilibj/livewindow/LiveWindow.java

@@ -128,7 +128,6 @@ public class LiveWindow {
    * <p>Actuators are handled through callbacks on their value changing from the SmartDashboard
    * widgets.
    */
-  @SuppressWarnings({"PMD.CyclomaticComplexity", "PMD.NPathComplexity"})
   public static synchronized void updateValues() {
     // Only do this if either LiveWindow mode or telemetry is enabled.
     if (!liveWindowEnabled && !telemetryEnabled) {