[native] modify SecondaryDeviceQRCodeScanner to support emulator

Summary:
if we're trying to add a device from an iOS simulator or android emulator, we should just display a TextInput where the user can paste the QR Code URL

Depends on D12619

Test Plan:
if device is added successfully, we are navigated back upon acknowledging the alert

if device can't be added successfully, we are navigated back when the alert fires

adding keyserver as secondary device works -- appears in DDB correctly

Reviewers: ashoat, bartek

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12620