[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.

wpilibc/src/main/native/cpp/smartdashboard/MechanismRoot2d.cpp

@@ -20,14 +20,12 @@ void MechanismRoot2d::SetPosition(double x, double y) {
 }
 
 void MechanismRoot2d::UpdateEntries(std::shared_ptr<nt::NetworkTable> table) {
-  std::scoped_lock lock(m_mutex);
   m_xEntry = table->GetEntry("x");
   m_yEntry = table->GetEntry("y");
   Flush();
 }
 
 inline void MechanismRoot2d::Flush() {
-  std::scoped_lock lock(m_mutex);
   if (m_xEntry) {
     m_xEntry.SetDouble(m_x);
   }