init method
initialise the service with details from app
Implementation
void init(String rootDomainFromApp, int rootPortFromApp) async {
atClientManager = AtClientManager.getInstance();
_atsign = atClientManager.atClient.getCurrentAtSign()!;
rootDomain = rootDomainFromApp;
rootPort = rootPortFromApp;
allContacts = [];
atContactImpl = await AtContactsImpl.getInstance(_atsign);
await fetchGroupsAndContacts();
}