Consistent use of method to determine component of report audience

Noticed while fixing MDL-76812 - we can improve the code to determine the component of a report audience (that being the plugin/subsystem in which the audience class resides) by removing the duplication we have:

1. In the audience cards exporter; and
2. In the audience class helper

& instead calling the appropriate method of the base class. This method is currently unused, and I think is poorly named/documented as to it's purpose, so we should find a more descriptive name for it at the same time