Always set a avb salt for hermetic build

When building images via `m` , build_image.py is invoked directly
without going through add_img_to_target_files. To ensure images built in
either way are identical, move uuid/salt computation to build_image.py,
so that the same uuid/salt will be used.

Bug: 281960439
Test: m installclean && m && m target-files-dir , maks sure images in
$OUT and $OUT/obj/PACKING/target_files_intermediates are identical

Change-Id: Icdab29df84f5a0ec7c080f99f9fdbdc3c9b10b90