[Content] Attributes 'modifiedDate' and 'releaseDate' fix in Content API

Add conversion from 'magic' database format to JS Date for given fields.

BUG=XWALK-2803
nit,
1. please add comment why need this conversion.2
2.  I will rewrite code as below to avoid big code segment(not so long in this case, but better to have) in condition block.

```
// Tizen video meta APIs return date format "%Y:%m:%d %H:%M:%S", 
// while standard JavaScript Date accept format as "%Y-%m-%d %H:%M:%S". 
// So we need this conversion function.
std::string ConvertToJSDateString(const char* date) {
  if (strptime(date, "%Y:%m:%d %H:%M:%S", &result) != NULL)
    return "";

  char output_date[20];
  memset(output_date, 0, 20);
  if (strftime(output_date, 20, "%Y-%m-%d %H:%M:%S", &result))
    return std::string(output_date);

  return "";
}
```