Use native implementation of getUserMedia on iOS 11+

[macdonst]

In iOS 11 Apple has included a native implementation of getUserMedia. We should detect that the OS already has an implementation and use it instead of our implementation.

We should be able to use something like:

cordova.require('cordova/modulemapper').getOriginalSymbol(window, 'navigator.mediaDevices');

to detect if navigator.mediaDevices existed before we clobber it with our implementation.

Once we have a working implementation on iOS 11 we should test ImageCapture and MediaRecorder.

[maverickmishra]

navigator.getMediaDevices is not available for WKWebView and UIWebView in iOS 11. It is only available for Safari.

[sandstrom]

This radar entry may be relevant: http://www.openradar.me/33571214

[stephanemagnenat]

I have tested on iOS 12, and it seems that WKWebView still does not ship navigator.mediaDevices.