transport: Don't put a capsfilter after the audio decoder and link the depayloader

We can't just force the audio decoder to output something specific, this will
fail with most audio decoders out there... and if conversion is required we
will do that later already.

Also make sure the pad that is ghosted is the decoded_valve and not the
capsfilter, which is incorrect.
Add test utilities

Run ./out/<platform>/bin/test-self-view after building to run a simple
audio/video capture and output test.

Run ./out/<platform>/bin/test-send-receive after building to run a test
with one sender and one receiver in the same application.