[native] Introduce RegistrationTile and fill in KeyserverSelection

Summary:
This diff styles KeyserverSelection to match what's in Figma, but without any selection state or "Next" button.

I also introduce two new components: KeyserverTile and KeyserverTileHeader. They serve to factor out the tile JSX, and in later diffs will also hold the selection state.

Depends on D7575

Test Plan:

before	after

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7576