Non UTF-8 reported encoding trips up SimpleXML

The XML files from IM report that they use `UTF-16` encoding despite actually being encoded as `UTF-8`. This trips up `simplexml_load_string()` in the `XML_Files::parse_item()` method.

For this particular migration, I've overrwritten the `Files::get_file_contents()` method in the importer class and performed a string replacement:

```php
$contents = str_replace( 'encoding="utf-16"', 'encoding="utf-8"', $contents );
```

HMCI should include a verification that UTF-8 is in use in the imported XML file.

Start with XML_Files::parse_item() and trace how it passes imported contents to simplexml_load_string(). Review the existing Files::get_file_contents() workaround as context, then verify handling for XML that declares UTF-16 while containing UTF-8 data; done means this input no longer trips the parser.

Written by the indexing model from the issue text.