[Unity] Fix Car API

[rajat2004]

Set API enabled so that it uses API controls rather than keyboard
Plus fix some warnings

Will be easier to review commit wise

Closes #2679, closes #2636, closes #2631

Some more cleanup needs to be done, keyboard_controls_ is actually incorrect and doesn't contain the keypresses, that is actually been done here. It can be seen that the external controls are applied only when isApiEnabled is true, which is being added in this PR.
CarPawn is supposed to provide the keyboard_controls but is just a placeholder, and doesn't do anything, so I think that can be removed

TODO:

• A method should probably be added so that CarPawnSimApi can fetch the keyboard controls from Car.cs, without this, the getCarControls API won't work to fetch the keyboard values.
• Figure out why reverse throttle isn't working (Works in Unreal)

airsim_unity_car_api.mp4

These changes will diverge Unity implementation from Unreal one, but shouldn't be much of a problem

[rajat2004]

@madratman @saihv @jonyMarino If possible, could you have a look at why Travis is not triggering and the Azure Pipelines status aren't getting updated? Seems to be happening for all PRs in the last 2-3 days
Thanks

Edit: Travis does get triggered, but not appearing in the CI checks - https://travis-ci.org/github/microsoft/AirSim/pull_requests

[HackBrettHB]

Thank you, this PR solved my problem! Why is it not merged into master yet?

[zimmy87]

Just some minor style changes. Tested with PythonClient\car\hello_car.py, PythonClient\car\drive_straight.py, and PythonClient\car\car_monitor.py, and the PR appears to work as intested

[zimmy87]

Thanks for the contribution @rajat2004!