[automerger] [DO NOT MERGE] Update Security String to 2018-04-01 on mnc-dev...

[automerger] [DO NOT MERGE] Update Security String to 2018-04-01 on mnc-dev Bug:73240847 am: e4d86d9b am: 83709d31 am: 70ea788d skipped: ac143721 am: 83c44812 am: 50abbfdb am: 00a67dda am: 2afc6907 skipped: 49f77737 am: b0aa7c34 am: a26a50ba am: 2d955bd7
am: 90b42aa3  -s ours

Change-Id: I7a794b7d11e41d8e73b050f1292d9b6b0df6400b