Add album art to media notification and lock screen

Summary:
Adds the album art to the media notification and lock screen.

How it looks on my phone (disregard the somewhat weird look, that's just the weird UI my phone vendor uses):

Google images shows how it'll look on other phones (this one includes the lock screen image):

And Android Oreo automatically colors the notification based on the album art color (untested, but should work):

Test Plan: Changes are minor and it works fine for me.

Reviewers: KDE Connect, nicolasfella, albertvaka

Reviewed By: KDE Connect, nicolasfella, albertvaka

Subscribers: nicolasfella

Differential Revision: https://phabricator.kde.org/D10798