Move libprotobuf-cpp-full and libprotobuf-cpp-lite to mixed builds denylist.

These libraries can be built by bazel, but not in the target and product
variant as required in mixed builds. See associated TODO for more info.

Bug: 206538317
Bug: 204810610
Bug: 204811222

Test: b build //external/protobuf:all
Change-Id: I45802c682dd40874de6393708d27323152463db6