Bug fix callJsMethod: Encode/decode JSON when passing data between languages

[gabrc52]

Fixes #76

This should also allow passing maps, lists, ints, and doubles between languages but only on Android.

On iOS, it's possible to pass them as parameters to JS functions. But returning lists and maps from JS to Dart will return odd strings generated by Swift/ObjC. I decided to keep the old (but not buggy with strings anymore) behavior here, because type information is lost (i.e. it doesn't distinguish between a 42 and a '42'). So something further should be done, like statically allowing only Strings or doing manual parsing.

https://pub.dev/documentation/webview_flutter/latest/webview_flutter/WebViewController/evaluateJavascript.html:

[gabrc52]

This is a breaking change on Android when non-Strings are returned from JS.