[native] call identity wallet login from ExistingEthereumAccount

Summary:
if usingCommServicesAccessToken is set, we should use identity wallet login here instead of the legacy keyserver call

Depends on D11016

Test Plan: successfully logged in to identity service from ExistingEthereumAccount. also tested that the alert displayed when identity service was down during login attempt

Reviewers: ashoat, inka, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11100