Export BOARD_AVB_<image>_ADD_HASHTREE_FOOTER_ARGS to soong

For use in the soong-generated partition images.

Also only update the build fingerprint file if its contents have
actually changed, as we're adding a dependency on it in soong code
and it causes files to be rebuilt every build without this change.
In make, there is no dependency added, because the file that uses it
is in a folder that's expected to be cleared every build with
`m installclean`. But in soong, the file that uses it is in
out/soong/.intermediates, so it actually needs that dependency to be
rebuilt.

Bug: 381120092
Test: m out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_system_image/android_common/prop, diff with make's prop file
Change-Id: I06917c895414bacc71ff06e92afff6d722dc9055