update_verifier: correct group in rc file

update_verifier should be in the cache group, not 'class'.

Also use PLOG instead of LOG if care_map.txt cannot be opened.

Bug: 36818743
Test: boot sailfish
Test: fake OTA on sailfish and verify update_verifier reads care_package

(cherry picked from commit 3a8002f8)

Change-Id: I7e4cccd457ee84054164077c81d04ad7cb394c7a