[native] Factor RegistrationTextInput out of KeyserverSelection

Summary: We'll need to introduce TextInputs for username and password selection. This diff extracts the TextInput code out of KeyserverSelection so it can be reused.

Test Plan:
Play around with the KeyserverSelection screen and make sure things work as expect:

1. Try focusing / bluring it
2. Make sure we're able to programmatically focus / blur it
3. Make sure it still looks the same
4. Make sure the style on focus still looks the same

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7895