Car class has the following functions:
-------waiting to be verified by Leyao Lee----------
- start motor when motor signal on firebase is "on"
- stop motor when motor signal on firebase is "off"
- start alarm when alarm signal on firebase is "on"
- stop alarm when alarm signal on firebase is "off"
- start remote camera when camera signal on firebase is "on" and the power signal on firebase is "off"
- stop remote camera when camera on firebase is "off" or the power signal on firebase is "on"
- take a theif picture when item in front of the distance sensor for more than 5 seconds
