snapuserd: use .ro num_verification_threads

We want snapuserd to use the .ro prop to configure number of threads
used during the verification phase. This has to be propogated through
metadata as early stage init doesn't have access to .ro props.

Bug: 332255580
Test: ota on cvd and look for logs
Change-Id: I3a1329fa4fd919e373a85039d6d6c8f7b4f6f483