Use SOONG_STUB_VENDOR_LIBRARIES for vendor linker.config.pb

STUB_LIBRARIES lists all stub libraries including "system" libraries.
There can be a library that provides "stub" only for "system" variant
while its vendor variant doesn't. (e.g. libz). In such cases, using
STUB_LIBRARIES will result a wrong "provide" list in
vendor/etc/linker.config.pb.

So, instead, use SOONG_STUB_VENDOR_LIBRARIES which only lists stub
libraries that are installed in /vendor.

Bug: 313806237
Test: m & cvd start
Test: $OUT/vendor/etc/linker.config.pb doesn't list libz.so
Change-Id: If0f2089594bda5f799fd495f80a27493553cb4e1