[DO NOT MERGE] Update security string to 2018-03-01 for mnc-dev Bug:71860241 am: fa4234e1df am: a615903311 am: 81e61008ed am: 51e7d3a175  -s ours am: cd6054fa86 am: 6619c472f3 am: a1c02ac99e
am: 2daf3f586c

Change-Id: I35d9356b70e92b50385a879e54576a244574dd1d