[native] Add explicit borderColor for non-selected RegistrationTile

Summary:
This wasn't noticeable in dark mode, but is much more noticeable in light mode. Since we set a borderWidth, React Native color sets a border. I'm not sure how it determines a default color for that border, but I think it just picks black.

Depends on D7888

Test Plan:

	before	after
light mode (not accessible; just showing for context)
dark mode (looks mostly the same)

The only change in dark mode is that the tile looks slightly larger in the "after" version, since we're using the backgroundColor of the tile as the borderColor, instead of using a more black backgroundColor (which fades into the background).

Reviewers: ginsu, atul, ted

Reviewed By: atul, ted

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7889