product variables are consumed via soong injection

Currently the list of product variables and whether they are arch-variant is
hard-coded. Instead, we can generate this data from the Soong Product_variables
struct and use soong_injection to consume it in Bazel.

Bug: 209801976
Test: m bp2build && build/bazel/ci/mixed_libc.sh
Change-Id: I8a91030950407a10832765f15eb010899d73da26