[native] Minor updates to UsernameSelection

Summary:
Two things I noticed while working on PasswordSelection:

1. The same returnKeyType="go" change that I included in the previous diff (D7989).
2. When checkUsernameValidity is called, if the username is valid we will now clear the usernameError.

Depends on D7989

Test Plan:

1. For returnKeyType="go", I confirmed the button was changed. For a visual indication of the change, check the previous diff.
2. For the checkUsernameValidity change, I triggered a username error by typing in an invalid username (eg. starting with a - character). Then I focused the keyboard, fixed the error, and then dismissed the keyboard. I confirmed that the error text disappeared after the dismissal.

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7990