device-manager: Actually fall back to reading from tty if no xkb layout specified

device-manager: Actually fall back to reading from tty if no xkb layout specified

Commit 7fcfcdf2d63025151a160f7948ee7406f0f16843 tried to fall back to
reading from a tty if no xkb layout was specified, but neglected to
actually check if no xkb layout was specified and force fall back to
reading from a tty in that case.

This commit changes device manager to actually disable reading from
/dev/input if the user doesn't specify a layout to use in vconsole.conf.

Closes https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/199