[native] use findUserID RPC in UsernameSelection component

Summary:
If usingCommServicesAccessToken, we should call the identity service RPC instead of the keyserver exactSearchUser API

if the username is reserved or registered on the identity service, we should set the username_taken error

Test Plan: enabled usingCSAT and tried to register an already registered and a reserved username, both failed with the username_taken error

Reviewers: ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11320