[automerger] [DO NOT MERGE] Update Security String to 2018-04-01 on mnc-dev Bug:73240847 am: e4d86d9b06 am: 83709d3152 am: 70ea788dfa skipped: ac14372129 am: 83c448128c am: 50abbfdb53 am: 00a67ddab1

Change-Id: I5bda5bccef672c7e713545801701481163d81734