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.
Merged-In: I8b47ac12d2e132f641129c9549ed22c3971d6c89
Change-Id: I8b47ac12d2e132f641129c9549ed22c3971d6c89
(cherry picked from commit ef1ff3de)
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>