Switch does not match React Native implementation

[Arlen22]

In React Native the thumb is one color, and the track changes color when switched, and this is controlled by two properties. However, in React Native Web, there are 4 properties, at least according to #832. However, nothing is listed in the Readme about Switch being any different from React Native. Even though I like this idea, React Native Web should rather match React Native in order to avoid edge cases.

[Arlen22]

Update: after playing around with the colors a little, I decided that the React Native system of keeping the same thumb color and changing the track color between a neutral color and a tertiary color looked about the nicest.

[necolas]

aa8593b