Disables: Autofill for sign up and username screens

[AjeshRPai]

Parent Issue https://github.com/wordpress-mobile/WordPress-Android/issues/16093

PR in parent app - https://github.com/wordpress-mobile/WordPress-Android/pull/17030

Description

For more details about the change see the p2(internal reference) paqN3M-Gp-p2

Testing Instructions

• See Parent PR for instructions