Refactor the native interface in preparation for switching iOS to use WKWebview.
* Finally rename the folder for device specific interfaces as `tablet` instead of `iPad`
* Update `import` statements to use the new name
* Create new `iOS.js` and `Android.js` based on previous `iPad/iOS.js` to separate the interfaces
* Add new `OS.js` class to manage the class variables, initialize the device interface delegate methods to the correct interface.
* refactor how `utils/lib` detects the current platform based on `navigtor.userAgent` based on https://stackoverflow.com/questions/37591279/detect-if-user-is-using-webview-for-android-ios-or-a-regular-browser. previous method relied on the Android interface being loaded or not. It can be difficult to detect the difference between in a browser and in a webview, but for now ScratchJr doesn’t need to worry about running in a browser
Added instructions to the README for how to get the google services configuration files for analytics. Removed (and ignored in future) the empty config files. Note: empty folders are ignored in git, so added an empty file `.exist` to the android folder so that it will be there as a place to hold the actual google services file.
Finally added the xCode scheme for the app that you need for newer versions of xCode.
