[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

Change-Id: I2bfc125bbc7cf0f23c90b82d4b739d66b8de24b8