Fix VNDK-Ext ABI check regression

VNDK-Ext are modules with `vndk.enabled: true` but not having
`vendor_available: true`.  In addition, VNDK-Ext should be checked by
source ABI checker.  This change fixes the regression introduced in

Bug: 110142940
Test: Create libminijail_ext, break some ABIs, and see an error.
Change-Id: I8b47ac12d2e132f641129c9549ed22c3971d6c89