[pm] fix get-domain-verification-agent command on aosp builds

The V2 domain verification agent is not available on AOSP builds. In
that case, we should return the V1 agent which is the intent filter
verification agent. This CL fixes the shell command results accordingly.

FIXES: 324658473
Test: manual; also with PreVerifiedDomainsTests

Change-Id: Ia9fd389b1dd1ed747cf4761b1a70e19ee763cd36