Multiply accelerometer values by -g on Windows

Windows implementation of device-motion methods returns acceleration, measrued in g's (source: https://msdn.microsoft.com/en-us/library/windows/apps/windows.devices.sensors.accelerometerreading.accelerationx). To be consistent with current plugin's specs, it is necessary to multiply accelerometer result by G constant (9.81 according to https://github.com/apache/cordova-plugin-device-motion/blob/master/doc/index.md#acceleration)