hide "add second device" instructions when transfer has started

[r10s]

when the "add second device" transfer started, the instructions should be hidden; they are no longer helpful but confusing, looking as if there is more to do than to wait for finishing :)

[Simon-Laux]

for the one implementing it: only hidden in transferring step (also center the progress bar), it should still be shown in the preparation step that is shown before the qr code