[native] Change border color of KeyserverSelection text input when focused

Summary:
Going through the designs again, I noticed that in some cases the text input had a lighter border color, but in other cases it was darker. I surmised that this has to do with selection state.

The text input here is now sufficiently complicated that I should probably factor it out. I'll do that in a subsequent diff, right before we need to reuse the component for password / username input screens.

Depends on D7889

Test Plan:

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7890