Add linker.config.pb support to android_filesystem

As part of the mk->bp conversion, all modules and partitions will
eventually be built with Soong. vendor.img (built with kati) uses some
rules in build/make/core to install a /etc/linker.config.pb file. This
CL adds this logic to `android_filesystem`. This soong module will
eventually be used to build vendor.img

There are two main inputs to linker.config.pb generation for vendor.
1. PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS, a list of json files
2. List of stub libraries installed in vendor

(1) will be passed to `android_filesystem` as `Linker_config_srcs`.

(2) has a subtle difference between kati and soong implementation. Kati
uses `SOONG_STUB_VENDOR_LIBRARIES` to determine the list of all vendor
stub libraries in the tree, and then uses `--system $TARGET_OUT/vendor`
to filter in the libraries which are actually installed. For the Soong
implementation, this will be replaced with ctx.VisitDirectDeps, followed
by child.HasStubVariants

Test: go test ./filesystem
Bug: 375686533

Change-Id: I6f9130d2aa866dcac9272b71939e40ed50a952ac