bluetooth: Move codec specific bits to their own respective files

We move the codec specific bits to their own respective files and now
make the codec specific initialisation use a GstBin, which the generic
GStreamer module now uses in the pipeline.

It is job of the codec specific function to add elements in the GstBin
and link the added elements in the bin. It should also set up the ghost
pads as a GstBin has no pads of it's own and without which the bin
cannot be linked to the appsrc/appsink.

Also, we now only initialise either the encoding or the decoding
pipeline and not both. The codec init API already gets passed the
for_encoding flag. We pass and use the same to codec specific init
functions.

Part-of: <pulseaudio/pulseaudio!440>
3 jobs for a2dp-codecs-v2 in 5 minutes and 6 seconds (queued for 1 second)