Use full camera preview when scanning QR codes
The QR code scanner only uses the upper half of the camera preview, as this is the only part that's visible when adding contacts in person. However, when pairing a mailbox the whole preview is visible, so the QR code may fail to scan if positioned in the middle of the preview.
We should either add a flag to control whether the whole preview is used, or (if testing on bad device combinations allows this), use the whole preview in both cases.
An example of a bad device combination is the Huawei Ascend Y330 showing the QR code and the Moto E3 scanning it.