BUILD_NUMBER: Use AOSP convention for unofficial builds

* Let's try to match things up

Per AOSP:
NOTE: When no BUILD_NUMBER is set by the build system, it defaults to a build
that starts with "eng." to signify that this is an engineering build and not
destined for release.

Ref: https://cs.android.com/android/platform/superproject/+/refs/heads/master:frameworks/base/services/core/java/com/android/server/pm/PackageManagerServiceUtils.java;l=1359

Change-Id: I81406ef28302d8e136cfcf4b270b2b5facedb1a5

release/version.sh

@@ -44,7 +44,7 @@ get_build_number() {
   if [[ -n ${OFFICIAL_BUILD} ]]; then
     echo "${version}"
   else
-    echo $((${version} - 1))
+    echo "eng.${version}"
   fi
 }