[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 am: 2afc690751 skipped: 49f77737a2

Change-Id: I45b510b29678883a6d89fd63960df9df4c84f9f5