Fix object to primitive conversion

[MarkusJx]

Currently, when an object is converted to a string using

const ArrayList = importClass('java.util.ArrayList');

const list = new ArrayList();
const val = list + '';

The value isn't properly converted to a string using the Java .toString method.
Also, the toString method shouldn't return a Promise<string>, it should return a string.
Therefore, the toStringSync method is useless and should be replaced by a toStringAsync returning a Promise<string>.