refactor(AudioRecord): rely on audio_common helpers and harden POSIX runtime

Align AudioRecord with shared audio_common/functestlib helpers and reduce
local logic that can drift.

- use pw_set_default_source helper instead of raw wpctl set-default
- ensure alsa_pick_virtual_pcm comes from audio_common.sh (no local copy)
- replace expr-based counters with POSIX arithmetic expansion
- keep existing CLI/behavior and result/log layout unchanged

No functional change to the recording matrix/config logic beyond robustness.

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

Author: smuppand