[SystemUiController] Add Window.isNavigationBarContrastEnforced APIs

[joharei]

Fixes #317.

I'm not sure about the naming of the APIs.

Also, is there is a better way to test this on different system gesture modes?

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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[joharei]

@googlebot I signed it!

[chrisbanes]

LGTM, just 2 small nits