main/postmarketos-base-ui: Add libao device override to pulseaudio subpackage

This merged request adds a new pine64 device specific subpackage for pulseaudio. It pulls out the existing pulseaudio config file and adds a new libao.conf file that fixes issues with playback using libao on the pinephone.

Switched the MR to update the postmarketos-base-ui-pulseaudio subpackage with the libao.conf file that specifies the default alsa device to use for playback.

For some more context see: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/23727 And this: https://gitlab.alpinelinux.org/alpine/aports/-/issues/12885

This is my first MR here so please feel free to let me know if I'm missing anything or could improve anything! Thanks! Adam