AudioTrackShared: Fix assertion on Java Offload

Two concurrent obtainBuffers are possible, one from the notification
thread and the other from the app data delivery thread.
Fix mUnreleased to prevent assertion on releaseBuffer.

Flag: EXEMPT bugfix
Test: atest AudioTrackOffloadTest#testPcmAudioTrackOffload
Test: atest CtsMediaAudioTestCases
Test: validated fix in the bug
Bug: 417174139
Change-Id: Ia4e687b5a8b860b8e6891d949703ed0d2b7cc0cb