Support file url album art - Android

Summary: If the album art is a file url, the code will ask the remote device to transfer that file. It's then stored in the album art cache.

Test Plan: Works fine for me, even when quickly skipping songs.

Reviewers: KDE Connect, nicolasfella, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, nicolasfella, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D11018