Unit tests and reference code for v15

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

util/src/string_conversions.cpp

@@ -292,4 +292,10 @@ int64_t htonll64(int64_t x) {  // Convert to big endian (network-byte-order)
   }
 }
 
+std::string BytesToString(const uint8_t* bytes, unsigned size) {
+  if (!bytes || !size) return "";
+  const char* char_bytes = reinterpret_cast<const char*>(bytes);
+  return std::string(char_bytes, char_bytes + size);
+}
+
 }  // namespace wvcdm