Reapply "Add support for product-specific build flags" am: 38b195ea

Original change: https://android-review.googlesource.com/c/platform/build/+/2816074



Change-Id: I4587ac35f3a24c36d875fdada7303f5846cac66a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>