Full-stack integrity: check vdex contents.

For priv-apps, if a vdex on /data has dex code, it must
be because the APK has compressed dex code.

Fall back to APK if not.

For test convenience, update --copy-dex-files to also
take a "always" variant.

bug: 30972906
Test: 677-fsi

Change-Id: I49f0eab46650145316a86de2971a3b9c140bcff7