utils: lib_gstreamer: deduplicate helpers and centralize caps inference/timeout/evidence

Refactor lib_gstreamer.sh to provide reusable building blocks for all Audio_* GST tests
(Playback) and remove duplicate/unused implementations.

Changes:
- Centralize gst-discoverer capture + audio caps inference (rate/channels)
- Provide a single gst-launch runner with timeout fallback (audio_timeout_run/timeout/direct)
- Consolidate backend evidence sampling with optional ASoC path_on fallback
- Keep backend-aware sink/source selection helpers for reuse by other audio runners
- Remove duplicated function variants and align naming/behavior to one canonical set

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

Author: smuppand