[native] Use consistent panel colors in QR code screens

Summary:
All of the components in qr-code-screen.react.js use a color from the panel color family, but the background in qr-code-sign-in-navigator.js use a color from the modal family. This diff just swaps the color out to instead remain consistent and use a color from the panel family.

This should have no impact visually because panelBackground is the same as modalBackground (designSystemColors.shadesBlack95)

Test Plan: Confirmed that there is no visual change

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10100