We only supported bp2build for rust_protobuf_host because the non-host toolchain for rust is not checked in yet. aosp/2759750 changed a converted module from rust_protobuf_host to rust_protobuf and broke CI.
This CL adds bp2build support for rust_protobuf but makes the target incompatible with android os for now untilnon-host rust toolchain is checked in.
Bug: 301956497
Test: b build //build/make/tools/aconfig:all --config=android
Change-Id: I739896c79f32674000c2603e394f16860a6fc57d
