[sim] Use DutyCycleEncoder FPGA index for sim device number (#2803)

The source channel doesn't necessarily correspond with the HALSIM device
index.

wpilibc/src/main/native/cpp/DutyCycleEncoder.cpp

@@ -132,6 +132,10 @@ void DutyCycleEncoder::SetConnectedFrequencyThreshold(int frequency) {
   m_frequencyThreshold = frequency;
 }
 
+int DutyCycleEncoder::GetFPGAIndex() const {
+  return m_dutyCycle->GetFPGAIndex();
+}
+
 int DutyCycleEncoder::GetSourceChannel() const {
   return m_dutyCycle->GetSourceChannel();
 }