audio: hal_play_test: Interactive streams test app fixes

Enable downmix & pan_scale control through kv_pair.
Store pan_scale & downmix params for each stream in
stream_config.
Update help menu to describe kv-pair for interactive
audio.

Change-Id: Ibe7e90bad7687f9f8427fda926382ee80ab08c09
CRs-fixed: 2091555