OptionalcloudOptionalcontextRequest context. Drives host selection: on Android the 2FA flow's
login-home context keeps this pre-session call on the bootstrap host
(b.i), matching the entrypoint.
OptionalscreenScreen instance id the entrypoint response issued for the picker
(LoginTwoFactorRequiredError.methodPickerScreenId). A fresh id is
generated only when the caller has none.
When set, sends
cloud_trust_tokeninclient_input_params(Android sendsnull).