Properly unlock on return in process thread

There is a return path in processAudioBuffer that did not
properly drop AudioTrack/AudioRecord object locks, leading
to deadlocks, which were only triggered upon certain lifetime
races.

Test: atest AudioTrackTest, atest AudioRecordTest
Bug: 230312538
Change-Id: I1d48061eca67ef7d5908ae8bd5186cbb3598f149