Selection: DynamicMultiSelectPlugin uses wrong constant to read provider name

There is a discrepancy between DynamicMultiSelectPlugin and DynamicDropdownPlugin on how they read the provider name from config.

DynamicDropdownPlugin used to use IValueListProvider.SERVICE but that got changed in HIPPLUG-531 to use Config.VALUELIST_PROVIDER

DynamicMultiSelectPlugin still uses IValueListProvider.SERVICE:
final IValueListProvider selectedProvider = context.getService(config.getString(IValueListProvider.SERVICE),
IValueListProvider.class);