[native] Introduce UsernameSelection screen

Summary:
This screen will be used for selecting a username for non-ETH accounts. Eventually we'll also allow ETH accounts to set a username.

This diff just sets up the scaffolding for the page. Later diffs will add functionality.

Depends on D7895

Test Plan:
Here's what it looks like now:

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7896