[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
am: ba60cad7ab  -s ours

Change-Id: I8c98113547529e4db2146eaa682e829ee0ac9fd2