Set AddPodcastActivity public

This fixes the IllegalAccessException when entering the add podcast
activity.
Hide logo downloads in download manager
Re-enable download and play position episode actions
Do not cache locally added podcasts

This lead to a situation in which podcasts were incorrectly set as
locally added because getLocallyAddedPodcasts actually returned all
cached podcasts in addition to locally added ones. This in turn lead to
invalid communications with gpodder.net.