Show art downloaded every time Phosh Media Widget is shown

Steps to reproduce

1. Using gnome-podcasts on Phosh (pinephone/librem 5), play a podcast
2. touch the top bar to invoke the media widget
3. Phone UI hangs until show art is downloaded

Show art is downloaded every time. This bug is being fixed in Phosh phosh issue 583 but downloading the show art each time is apparently an issue with the player itself. More details can be found in the phosh issue linked.

Current behavior

gnome-podcasts provides show art to phosh as an http/https URL, not a file stored on disk.

Expected behavior

Show art provided would be stored on disk, not downloaded from the Internet every time a media widget is shown.