audio: Fix observable/hardware position check in VTS

Unify logic for checking the observable and hardware
position returned by the HAL. Fix the problem that
the reported hardware position was not checked for
being `UNKNOWN`.

Bug: 363139283
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Id3beab1d1401a9608ad7e604a800ae19873270bf