Fix AudioTrack shared memory leak

Bug: 2801375
Change-Id: I50e15164fe310f69ea019dca5b49171a02bc6992
diff --git a/services/audioflinger/PlaybackTracks.h b/services/audioflinger/PlaybackTracks.h
index 0308b99..f7ad6b1 100644
--- a/services/audioflinger/PlaybackTracks.h
+++ b/services/audioflinger/PlaybackTracks.h
@@ -117,7 +117,10 @@
     enum {FS_INVALID, FS_FILLING, FS_FILLED, FS_ACTIVE};
     mutable uint8_t     mFillingUpStatus;
     int8_t              mRetryCount;
-    const sp<IMemory>   mSharedBuffer;
+
+    // see comment at AudioFlinger::PlaybackThread::Track::~Track for why this can't be const
+    sp<IMemory>         mSharedBuffer;
+
     bool                mResetDone;
     const audio_stream_type_t mStreamType;
     int                 mName;      // track name on the normal mixer,