bluetooth: Generalize caps setup for GStreamer transcoders

⚠ This MR is based on top of !473 as it contains many suggestions relevant to AAC.

@SanchayanMaity and me discussed elsewhere that it's nice to have a single GstBin for a codec-specific implementation to put its elements in, allowing the code to remain rather generic. However, this exact same codec-specific initialization was heavily interacting with its prospective peer - the appsrc/sink, effectively breaking that boundary.

This MR removes that dependency by using capsfilter. At the same time caps setup for the sink/src interacting with PA has moved to the common initializer.