Unit tests and reference code for v15

This CL updates the unit tests and reference code to support OEMCrypto
API v15.

util/include/string_conversions.h

@@ -26,6 +26,7 @@ std::string HexEncode(const uint8_t* bytes, unsigned size);
 std::string IntToString(int value);
 int64_t htonll64(int64_t x);
 inline int64_t ntohll64(int64_t x) { return htonll64(x); }
+std::string BytesToString(const uint8_t* bytes, unsigned size);
 
 }  // namespace wvcdm