[v20.05] phosh: make volume buttons work
All patches are submitted upstream and basically accepted by the lead maintainer
https://source.puri.sm/Librem5/phoc/-/merge_requests/166
https://source.puri.sm/Librem5/phosh/-/merge_requests/537
Volume buttons work on all screens (locked, screen off, screen on) and adjust the audio volume (who would've thought)