Initialize should return OK instead of ERROR when calling multiple times

The initialize call should be idempotent and return on multiple calls the same result or you should not return a json object with `isSdkAvailable`.

Also changed misleading parameter name `isSdkAvailable` to `isSandboxMode`. See documentation for further information: https://developer.amazon.com/public/apis/earn/in-app-purchasing/javadocs/in-app-purchasing-api-reference