android: MarkStable: mark android_build_branch as optional

BUG=b:187888777
TEST=generate.sh

Change-Id: Ia978b8eeaf405de8cf4886e4dfb8502b50b063c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2891292
Auto-Submit: Shao-Chuan Lee <shaochuan@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
2 files changed