snapuserd: use verification_block_size

verification block size will be read from vabc_features.mk. In order for
snapuserd to have access to this value in first stage init, we will
persist the data to metadata protobuf

Bug: 332255580
Test: full ota on cvd logging the verification block size
Change-Id: I03ef8a6976ced9309efede30c792aef786a0a745