libaudiohal@aidl: Fix handling of "unknown" position

In HIDL HAL, methods for returning the current position could
return 'ENOSYS' indicating that it can't be currently retrieved.
The AIDL HAL adapter was missing this behavior. Instead, it
was clamping the returned 'UNKNOWN' position to '0', causing
issues with frame accounting at the framework layer.

Bug: 374929507
Test: atest audiorecord_tests audioeffect_tests audiorouting_tests
Test: atest trackplayerbase_tests audiosystem_tests
Test: atest CtsMediaAudioTestCases
Change-Id: I24faa2fb4075ee0c5762a8780a5f6cb94697d961