AudioRecord: ensure documented behavior of active rec config

Make sure getActiveRecordingConfiguration() doesn't throw an
exception when queried after the recorder is released as this
isn't part of the API contract

Bug: 170608713
Test: atest android.media.cts.AudioRecordTest#testGetActiveRecordingConfiguration
Change-Id: I8003056e65ab0e125b5478d6c765878b2840d4b9