Add AirSimManager to CarDemo scene in Unity

[lnexenl]

Fixes:

The RPC server of CarDemo isn't launched when running CarDemo scene in Unity.

About

Add AirSimManager to CarDemo scene in Unity to launch RPC server when CarDemo scene is started.

How Has This Been Tested?

# when AirSimManager wasn't added to CarDemo, the rpc server was not started.
python .\hello_car.py
WARNING:tornado.general:Connect error on fd 1192: WSAECONNREFUSED
WARNING:tornado.general:Connect error on fd 1192: WSAECONNREFUSED
WARNING:tornado.general:Connect error on fd 1192: WSAECONNREFUSED
WARNING:tornado.general:Connect error on fd 1192: WSAECONNREFUSED
WARNING:tornado.general:Connect error on fd 1192: WSAECONNREFUSED
Traceback (most recent call last):
  File ".\hello_car.py", line 11, in <module>
    client.confirmConnection()
  File "C:\code\AirSim\PythonClient\airsim\client.py", line 139, in confirmConnection
    if self.ping():
  File "C:\code\AirSim\PythonClient\airsim\client.py", line 35, in ping
    return self.client.call('ping')
  File "C:\Users\*\anaconda3\lib\site-packages\msgpackrpc\session.py", line 41, in call
    return self.send_request(method, args).get()
  File "C:\Users\*\anaconda3\lib\site-packages\msgpackrpc\future.py", line 43, in get
    raise self._error
msgpackrpc.error.TransportError: Retry connection over the limit

# when AirSimManager is added to CarDemo, hello_car.py runs correctly.
python .\hello_car.py
Connected!
Client Ver:1 (Min Req: 1), Server Ver:1 (Min Req: 1)

API Control enabled: True
Saving images to C:\Users\*\AppData\Local\Temp\airsim_car
Speed 0, Gear 0
Go Forward
Go Forward, steer right
Go reverse, steer right
Apply brakes
Retrieved images: 4
Type 3, size 23651
Type 2, size 36864
Type 0, size 23596
Type 0, size 110592
Speed 11, Gear 0
Go Forward
Go Forward, steer right
Go reverse, steer right
Apply brakes
Retrieved images: 4
Type 3, size 1698
Type 2, size 36864
Type 0, size 26066
Type 0, size 110592
Speed 9, Gear 0
Go Forward
Go Forward, steer right
Go reverse, steer right
Apply brakes
Retrieved images: 4
Type 3, size 1720
Type 2, size 36864
Type 0, size 22335
Type 0, size 110592

Screenshots (if appropriate):

Compared to DroneDemo, CarDemo lacks AirSimManager.

[ghost]

All CLA requirements met.

[zimmy87]

Tested this fix locally and it's working for me, so I am moving ahead with merging.

[Lyccccccccccc]

Hi, I don't know what's wrong but when I added AirSimManager to CarDemo and hit play, Unity just crashed.