Export VBMETA_VENDOR props to soong

vbmeta_vendor is a chained partition of vbmeta.img. fsgen uses
PartitionQualifiedVariables to determine which partitions should be
added to Chained_partitions of the autogenerated vbmeta.img. The absence
of vbmeta_vendor was leading to AVB related boot failures when flashing
a local build for husky.

Bug: 419860251
Test: rm -rf out/target/product/husky
Test: m --soong-only
Test: vendor/google/tools/flashall -w
Change-Id: I073e14a56f1a09b3ee5b72c6783225e5970acc6e