system_properties.proto: rename update_verifier_nonencrypted to update_verifier

incident_helper contains some "interesting" code that automagically
fills in a SystemPropertiesProto by finding the protobuf fields by name
using system property name components.  One of these fields is
'update_verifier_nonencrypted' which is nested inside 'init_svc'.  This
gets filled in with the value of init.svc.update_verifier_nonencrypted.

The "update_verifier_nonencrypted" service is being renamed to simply
"update_verifier", and as a result the name of the corresponding
"init.svc.${service_name}" system property will change too.  Therefore,
rename the corresponding protobuf field too.

Bug: 208476087
Test: presubmit
Change-Id: I782ab726870df64c5b9cb354b223634d85a0f63b