Now maximizes QR code width to the screen while keeping its ratio. Not
optimal for screen configs where the space available to display the code
itself horizontally > vertically (including landscape, which is also not
optimal for other reasons).
The problem was that older Android versions took the 0dp value
seriously, actually rendering the FrameLayout with 0 height (→ it was not
visible at all). There seems to be a slight inconsistency in how
RelativeLayout interprets width and height values when below and above
or before and after are set.