Add standalone list_products and list_variants commands. am: c6e5a456

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/3108309



Change-Id: Icbb61c36a3dff0e74d13f6e6067679a48d003bfb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>