Restore AOSP-style version numbers - DO NOT MERGE

Change-Id: I2c81a4dacc13e4705388c3d359410c81999d7b95
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index d87a032..f3b2297 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -41,7 +41,7 @@
   # which is the version that we reveal to the end user.
   # Update this value when the platform version changes (rather
   # than overriding it somewhere else).  Can be an arbitrary string.
-  PLATFORM_VERSION := 5.1
+  PLATFORM_VERSION := 5.1.51
 endif
 
 ifeq "" "$(PLATFORM_SDK_VERSION)"