Check the completeness of original blocks during target files validation

The validate_target_files.py checks the 'incomplete' field of the range
in file_map. And range has already considered the shared blocks and
could be smaller in size than the original file range. Therefore, the
'incomplete' flag was set on the original range in common.py; and we
should switch to use the original range also during validation.

I also checked another flag usage in CanUseImgdiff(), and it has
explicitly rejected cases of shared blocks.

Bug: 124868891
Test: unit tests pass
Change-Id: I03959625d7b81fd83420db98f01d23f54064bcd2