[wpilib] Fix deadlocks in Mechanism2d et al. (#3770)

UpdateEntries() and Flush() are called from methods that lock the mutex,
so locking it again will cause deadlocks. This also updates the Java
code to make MechanismObject2d::update synchronized like in the C++
version.

wpilibj/src/main/java/edu/wpi/first/wpilibj/smartdashboard/MechanismLigament2d.java

@@ -128,7 +128,7 @@ public class MechanismLigament2d extends MechanismObject2d {
   }
 
   @Override
-  protected synchronized void updateEntries(NetworkTable table) {
+  protected void updateEntries(NetworkTable table) {
     table.getEntry(".type").setString("line");
     m_angleEntry = table.getEntry("angle");
     m_lengthEntry = table.getEntry("length");
@@ -138,7 +138,7 @@ public class MechanismLigament2d extends MechanismObject2d {
   }
 
   /** Flush latest data to NT. */
-  private synchronized void flush() {
+  private void flush() {
     if (m_angleEntry != null) {
       m_angleEntry.setDouble(m_angle);
     }

wpilibj/src/main/java/edu/wpi/first/wpilibj/smartdashboard/MechanismObject2d.java

@@ -52,7 +52,7 @@ public abstract class MechanismObject2d {
     return object;
   }
 
-  final void update(NetworkTable table) {
+  final synchronized void update(NetworkTable table) {
     m_table = table;
     updateEntries(m_table);
     for (MechanismObject2d obj : m_objects.values()) {

wpilibj/src/main/java/edu/wpi/first/wpilibj/smartdashboard/MechanismRoot2d.java

@@ -86,7 +86,7 @@ public final class MechanismRoot2d {
     return m_name;
   }
 
-  private synchronized void flush() {
+  private void flush() {
     if (m_xEntry != null) {
       m_xEntry.setDouble(m_x);
     }