Reapply "Add support for product-specific build flags" am: 38b195ea am: c8ebe4c2 am: d9e894e8

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



Change-Id: I1ac0c0fa11a906a5308bdbf8886be942382fca16
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>