releasetools: Fix the support for getting reproducible images.

We used to support getting idempotent images when given the same inputs,
by specifying UUID and AVB salt to the image builders. The support was
broken by Q changes that moved away from build-time fingerprints [1].

This CL addresses the issue by updating the logic in computing UUID and
AVB salt to be based on BuildInfo.fingerprint, as opposed to the
deprecated static build fingerprint.

[1] https://android-review.googlesource.com/c/platform/build/+/892933

Bug: 134525174
Test: TreeHugger
Test: lunch a target; `atest --host releasetools_test releasetools_py3_test`
Test: `m dist`; `unzip out/dist/target_files.zip IMAGES/\*`;
      `zip -d out/dist/target_files.zip IMAGES/\*`
Test: `add_img_to_target_files -a out/dist/target_files.zip`. Verify that
      the rebuilt images are identical to the ones deleted.
Change-Id: I8844fc7f044e9abfcd96a00e8958fa0583c70eaf