fun fetchUsersFromKtLib(callback: (users:List<String>) -> Unit) {
    val api = HeartbeatApi()
    GlobalScope.launch { 
        val users = api.fetchUsers()
        callback(users)
    }
}

Kotlin shared lib

fun fetchUsersFromKtLib(callback: (String) -> Unit) {
        GlobalScope.launch {
            val response = client.get<JsonObject> { url("$baseUrl/heartbeat.json") }
            val listOfUsers = response.keys.iterator()
            listOfUsers.forEach {
                callback(it)
            }
        }
    }

Kotlin class inside old java project

fun fetchUsers(listFromAndroid: ArrayList<String>) {
    val api = HeartbeatApi()
    return api.fetchUsersFromKtLib {
        listFromAndroid.add(it)
    }
}