pine64-pinephone: move shelli alsa cfg to subpkg

Move the shelli-specific alsa config from !1741 (merged) into a subpackage that only gets installed together with shelli.

With the config installed, programs using alsa instead of pulseaudio, can't be controlled through the volume setting in Phosh anymore (and probably other UIs too). Shelli doesn't use pulseaudio.

Change the path of the config file like in !1877 (closed), so it's easier to override it if necessary.

While at it, improve the APKBUILD slightly by fixing the install_if of the phosh subpkg (should depend on =$pkgver-r$pkgrel, see APKBUILD reference), and fix long lines.

Fix #892 (closed).