Fix bug in byte vector serialization

Byte vectors are used by the generated C++ code as the representation
of Java byte[].  This type is serialized as a packed byte array on
the java side.

Bug: 25012838
Test: integration tests for byte[] show this type crossing language
      boundaries correctly.

Change-Id: I8bb1b8ffcb77ced44f99f6b370226a32694f7df1