If no hls group name is given, we would default to "audio" but as
we are working to support text, this won't make sense to see
GROUP="audio" on a text stream.
Instead default to "default-audio-group" for audio streams. A specific
text default will be added later.
Bug: #205
Change-Id: Iefc8bfd35708286619d0004348294d98f2c38482