SettingsApp Implement DISALLOW_GRANT_ADMIN in user creation flow

In the process of creating a user through the Settings app, if the current user has the DISALLOW_GRANT_ADMIN restriction, the dialogue for granting ADMIN privileges will be hidden after this implementation. This action ensures that even if child users can create SECONDARY users, they cannot create ADMIN users.

Design doc: go/unicorn-hsum

Bug: 357819541
Test: manually verified the user is not allowed to create ADMIN user when DISALLOW_GRANT_ADMIN restriction is applied.
Flag: android.multiuser.unicorn_mode_refactoring_for_hsum_read_only
Change-Id: I4b40f83b50d4de0af103d4f5ca400e42345144e0