AudioTrack: fix position callback after restore

When restoring an AudioTrack, the next position callback point
should not be modified and set ahead of current buffer head.
Otherwise, as frames are dropped, the new position is never reached
and an application relying on position callbacks to reload the buffer
would be stalled.

Bug: 11868603.
Change-Id: I93b2a311642a0c89944b78bcc0482d4ceed98ae4