[examples] Ensure right side motors are inverted (#3836)

Fixes #3827
Adds MotorController inversion for right side, removes inversion in
setVoltage methods.

Also fixes various XboxController negations (was inconsistent throughout examples).

wpilibcExamples/src/main/cpp/examples/ArmBot/cpp/RobotContainer.cpp

@@ -17,8 +17,8 @@ RobotContainer::RobotContainer() {
   // Set up default drive command
   m_drive.SetDefaultCommand(frc2::RunCommand(
       [this] {
-        m_drive.ArcadeDrive(m_driverController.GetLeftY(),
-                            m_driverController.GetRightX());
+        m_drive.ArcadeDrive(-m_driverController.GetLeftY(),
+                            -m_driverController.GetRightX());
       },
       {&m_drive}));
 }

wpilibcExamples/src/main/cpp/examples/ArmBot/cpp/subsystems/DriveSubsystem.cpp

@@ -16,6 +16,10 @@ DriveSubsystem::DriveSubsystem()
   // Set the distance per pulse for the encoders
   m_leftEncoder.SetDistancePerPulse(kEncoderDistancePerPulse);
   m_rightEncoder.SetDistancePerPulse(kEncoderDistancePerPulse);
+  // We need to invert one side of the drivetrain so that positive voltages
+  // result in both sides moving forward. Depending on how your robot's
+  // gearbox is constructed, you might have to invert the left side instead.
+  m_rightMotors.SetInverted(true);
 }
 
 void DriveSubsystem::Periodic() {