[lib] implement QRAuthContext

Summary:
ENG-8385.

QR code login is not working right now. As described here (Point 3) and here the issue is occuring because of the fact that useQRAuth is unmounting before it can send SECONDARY_DEVICE_REGISTRATION_SUCCESS. This stack is refactoring the code to move logic to context (see here).

This wraps the platform-specific logic to a context. Usage later in the stack.

Depends on D12445

Test Plan: Tested later in the stack.

Reviewers: bartek, ashoat

Reviewed By: bartek, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12449