[native] Introduce UsernameSelection screen
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
Here's what it looks like now:
Reviewers: ginsu, atul
Reviewed By: atul
Differential Revision: https://phab.comm.dev/D7896