Rust cdylib/statliclib support for vendor snapshot.

Adds support for platform vendor_available Rust FFI libraries and
binaries to be included in the vendor snapshot.

Because rlib and dylibs are not yet in snapshots, libstd cannot be
included in a vendor snapshot. As a result, vendor-specific Rust code
can't be guaranteed to work with the platform-provided vendor_available
modules built with a newer toolchain. For now, a check is added
indicating vendor-specific Rust code is unsupported.

This changes the linkage for vendor variants of these modules to default
to rlib linkage since dylibs cannot be included in the snapshot yet.

Bug: 184042776
Test: m nothing # new Soong tests pass
Change-Id: I502eaa4bb962eb87ff868fcf49b435f0d2f982e6
diff --git a/rust/Android.bp b/rust/Android.bp
index b611672..6398390 100644
--- a/rust/Android.bp
+++ b/rust/Android.bp
@@ -53,6 +53,7 @@
         "rust_test.go",
         "source_provider_test.go",
         "test_test.go",
+        "vendor_snapshot_test.go",
     ],
     pluginFor: ["soong_build"],
 }