Preserve the monotonic property when subtracting 0 block

The monotonic property of the range is lost after the recent change to
subtract 0th block from the file range. As a result, we read the block
in the wrong order when pull files from the raw image. This yields a
incorrect hash during validation.

Bug: 150334561
Test: run validate_target_file. unittests
Change-Id: Id4fedaaf273c42c8d21a4549fda4c9a45e22066a