Include per-partition fingerprint as AVB prop.

This allows querying per-partition fingerprint via libavb (in
particular, avb_property_lookup).

Bug: 80097573
Test: `m dist`; `avbtool info_image --image /path/to/image` to check the
      written prop.
Test: `atest --host releasetools_test releasetools_py3_test`
Test: Run sign_target_files_apks to sign a target_files.zip that's built
      with the change. Check the AVB prop in the signed images.
Change-Id: Id4f06df82c29e77aca128439c8c11f0367fe587e