update: always include name/summary/desc in index.xml if available

With the new localization support, the name/summary/description in the
metadata file becomes the global override.  So most apps are not going to
have those fields present in their metadata file.  This fixes the index.xml
generation to fall back to the localized versions of those fields when they
are not set in the metadata field.

https://forum.f-droid.org/t/what-has-happend-to-osmand