releasetools: Skip validating non-sparse images.

Targets can define 'TARGET_USERIMAGES_SPARSE_EXT_DISABLED := true' to
generate non-sparse system images, but validate_target_files.py doesn't
work with such images. This CL adds a workaround to temporarily skip the
file consistency check for such images.

Bug: 79616357
Test: Run validate_target_files.py on a target_files.zip that's not
      using sparse image.
Test: Run validate_target_files.py on marlin target_files.zip (which
      uses sparse image).
Change-Id: I1f4066c5b3fec595b10cab10283d62c1c5a6c624