Reapply "Add support for product-specific build flags" am: 38b195ea am: c8ebe4c2

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



Change-Id: I9169d68e45df499126929e57b999242fb99339a7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>