debian: Install systemd service

squeekboard-restyled is not needed any more, because the user can alter the style in the systemd unit if desired.

This works with https://source.puri.sm/Librem5/debs/pkg-phosh/-/merge_requests/25

This supersedes !447 (closed)

The small difference from the previous attempt is that the package built here for testing doesn't care about phosh version.

This makes the removal of sm.puri.OSK0 startup thing possible: phosh!773 (closed)

This makes it possible to replace Squeekboard with other input methods within systemd sessions.