Dedupe logic to generate linker.config.pb

Between android_filesystem and generic_system_image.
`getLibsForLinkerConfig` will return a list of provideLibs and
requireLibs for the fileystem. `linkerConfig.BuildLinkerConfig` will
then filer out the non-stub libraries.

For `android_filesystem`, requireLibs is ignored for now to match the
logic in the kati built vendor.img

Test: m nothing --no-skip-soong-tests
Test: no diff in out/soong/.intermediates/build/make/target/product/generic/generic_system_image/android_common/gen/root-extra/system/etc/linker.config.pb
paste of cmd: https://diff.googleplex.com/#key=KAqqP9bhKZMD

Change-Id: I1f1d626a3a161fb2e12597909fd287533cbb8482