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/tools/buildinfo.sh b/tools/buildinfo.sh
index dcd9ab5..6cd9499 100755
--- a/tools/buildinfo.sh
+++ b/tools/buildinfo.sh
@@ -7,6 +7,7 @@
 echo "ro.build.display.id=$BUILD_DISPLAY_ID"
 echo "ro.build.version.incremental=$BUILD_NUMBER"
 echo "ro.build.version.sdk=$PLATFORM_SDK_VERSION"
+echo "ro.build.version.preview_sdk=$PLATFORM_PREVIEW_SDK_VERSION"
 echo "ro.build.version.codename=$PLATFORM_VERSION_CODENAME"
 echo "ro.build.version.all_codenames=$PLATFORM_VERSION_ALL_CODENAMES"
 echo "ro.build.version.release=$PLATFORM_VERSION"