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

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



Change-Id: Ida0d96a53c78a874a479a57129f0e137e569e781
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>