Only enable zucchini if configuration file exists

Only enable zucchini if configuration file zucchini_config.txt exits
in target files and version in source and target is the same.

Without this patch zucchini would be enabled if configuration file
was missing from both target files.

Bug: 231204699
Test: Run ota package generation with zucchini_config.txt missing from
      both source and target zips. Confirm that zucchini is disabled.
Change-Id: Ia34998fd911d5860b38fe49fa5a88056a22d661e