Initialize ro.build.version.preview_sdk

Set the initial (version 1!) value for the preview SDK version
readable by apps. If we're on a release version, force it to be 0.

Change-Id: Ib3e6cad1f59cea8e4a781827d4a35dd3620b90f1
diff --git a/core/Makefile b/core/Makefile
index b5cb727..0528b44 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -205,6 +205,7 @@
 			BUILD_NUMBER="$(BUILD_NUMBER)" \
 			PLATFORM_VERSION="$(PLATFORM_VERSION)" \
 			PLATFORM_SDK_VERSION="$(PLATFORM_SDK_VERSION)" \
+			PLATFORM_PREVIEW_SDK_VERSION="$(PLATFORM_PREVIEW_SDK_VERSION)" \
 			PLATFORM_VERSION_CODENAME="$(PLATFORM_VERSION_CODENAME)" \
 			PLATFORM_VERSION_ALL_CODENAMES="$(PLATFORM_VERSION_ALL_CODENAMES)" \
 			BUILD_VERSION_TAGS="$(BUILD_VERSION_TAGS)" \