Multimedia/GStreamer: Audio_Playback: add LAVA env defaults and shared lib runners

- Audio_Playback run.sh now supports LAVA-provided environment defaults and reduces inline duplication by using
  shared helpers from lib_gstreamer.sh.
- Add env-to-default mapping for all CLI params (AUDIO_* and AUDIO_PLAYBACK_*)
- Keep CLI as highest priority override
- Use shared gst-launch timeout runner and shared evidence sampling wrapper
- Use shared caps inference + caps forcing policy (user-requested or inferred only)
- Improve logging: prints normalized duration, backend chain, sink selection and pipeline

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand