vndk-def: Scan vdex files for dependencies

This commit scans vdex files for dependencies as well.  This commit is
necessary because some apk files or jar files are stripped, i.e. not
containing dex files.

Test: ./vndk_definition_tool.py apk-deps --system ... --vendor ...
Change-Id: I3f9009e0147d244c141f1f4e13369e23801a495a