[wpilib] Make protected fields in accelerometers/gyros private (#6134)

This avoids needing add redundant JavaDocs to them, and better reflects
how we design our modern classes (the classes modified here were around
with minimal changes since 2008 or so).

wpilibc/src/main/native/include/frc/ADXL345_I2C.h

@@ -100,7 +100,7 @@ class ADXL345_I2C : public nt::NTSendable,
 
   void InitSendable(nt::NTSendableBuilder& builder) override;
 
- protected:
+ private:
   I2C m_i2c;
 
   hal::SimDevice m_simDevice;