Improve handling of /dev/disk/by-id/ changes

Preserve previous answer to grub-pc/install_devices if we have to ask
grub-pc/install_devices_disks_changed and the user chooses not to
install to any devices, so that we can recover from temporary bugs that
cause /dev/disk/by-id/ paths to change.

Ideally this would also include explanatory text in the debconf
template, but it's a bit late in the buster release cycle for new
translatable text.  I've left an XXX comment for the time being.

Closes: #919029