main/postmarketos-artwork: enable lockscreen background for phosh

Achill Gilgenastrequested to merge
fossdd/phosh-lockscreen-bg into master

By default it is disabled by upstream[1], but if we use a wallpaper thats adapted to mobile (it is anyways) and g-c-c sets both wallpapers already.

If users want to change the lockscreen wallpaper only, they have to use p-m-s.

[1] https://gitlab.gnome.org/World/Phosh/phosh/-/commit/24a255401288e3793a3686bf5c605d1c55bb1176
20250514_18h59m15s_grim 20250514_18h59m20s_grim

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