TTRSS: Optimize get_subscribed_urls for api_level >= 2
TTRSS: Optimize get_subscribed_urls for api_level >= 2

Since api_level >= 2, we can request a list of all feeds by using
"getFeeds". That way, the feedlist can be acquired by just two queries,
instead of 1 + #categories.

der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat