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.

(cherry picked from commit f0fc52b5)

Change-Id: I0afd4c9655d642613fd9012c7c1e4e9fc4edd1db