[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

Change-Id: Ic6f9e8e25a106a49986683e8f23f641d0cc749f5