Use add_slot_suffix function in edify script

Whenever a device is retrieved from fstab, wrap it with
add_slot_suffix() if it has slotselect option.

Test: change fstab (changes boot image, which is a static partition),
      change system partition (a dynamic partition),
      generate incremental OTA with --force_non_ab and apply it on
      cuttlefish
Bug: 153581609
Change-Id: Id3f8e4425b65176baf1b0ff1ee07ab3d820a3a7f