[native][web] Fetch AES key and device ID in parallel in generateQRCode

Summary:
Also removed an unnecessary check on the device ID being set, since the Flow types indicate it should always be set.

Depends on D12108

Test Plan: I tested the flow using staging, with my phone as a primary device and a local simulator as a secondary device

Reviewers: bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12109