main/postmarketos-base-ui: add audio subpackages

This MR implements audio-pipewire and audio-pulseaudio subpackages for postmarketos-base-ui. This is required for the implementation for the proposal here: #2718.

To summarize the proposal: the goal is to make it where audio stacks are not hard-coded to UI packages. By making UI packages rely on postmarketos-base-ui-audio it will be easier to switch out audio stacks depending on user or UI choice.