Update Security String to 2018-03-05 for master

Bug:72469949
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 8a7b684..d0023bd 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -235,7 +235,7 @@
     #  It must be of the form "YYYY-MM-DD" on production devices.
     #  It must match one of the Android Security Patch Level strings of the Public Security Bulletins.
     #  If there is no $PLATFORM_SECURITY_PATCH set, keep it empty.
-      PLATFORM_SECURITY_PATCH := 2018-02-05
+      PLATFORM_SECURITY_PATCH := 2018-03-05
 endif
 
 ifndef PLATFORM_BASE_OS