Add standalone list_products and list_variants commands. am: c6e5a456 am: e971cfdb

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



Change-Id: Iae5819449cd7ca1d69e52972a07d9af2a0a3c791
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>