Update Security String to 2016-05-01 in preparation for May 2016 Security OTA

b/27724973

Change-Id: I572123586730e45b304f352c8b0fd6371ff5788b
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index b0602ed..0308bc1 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -73,7 +73,7 @@
   # Can be an arbitrary string, but must be a single word.
   #
   # If there is no $PLATFORM_SECURITY_PATCH set, keep it empty.
-  PLATFORM_SECURITY_PATCH := 2016-04-01
+  PLATFORM_SECURITY_PATCH := 2016-05-01
 endif
 
 ifeq "" "$(PLATFORM_BASE_OS)"