[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 am: 587ec653f1
am: fdac6439e4

Change-Id: I8af9ec19010e43bb1e8359af152543332f1499f8