Adds support for snapshotting Rust rlibs. This allows us
vendor-specific code that uses rlib-only linkage until dylib
snapshot support is added.
Bug: 184042776
Test: m nothing # new Soong tests pass
Test: Example test Rust vendor module builds
Test: m dist vendor-snapshot # includes rlibs
Change-Id: I4976d3e1efec0ee778cc97730d45be471dffb678
