Workaround for issue 2046783.

Apparently the problem is caused by the fact that A2dpAudioStreamOut::standby() calls a2dp_stop() after the headset has been powered down.
The workaround consists in indicating to A2DP audio hardware that a close request is pending and that stanby() must be bypassed.
diff --git a/libs/audioflinger/A2dpAudioInterface.h b/libs/audioflinger/A2dpAudioInterface.h
index d6709e2..35a6e11 100644
--- a/libs/audioflinger/A2dpAudioInterface.h
+++ b/libs/audioflinger/A2dpAudioInterface.h
@@ -112,6 +112,7 @@
                 Mutex       mLock;
                 bool        mBluetoothEnabled;
                 uint32_t    mDevice;
+                bool        mClosing;
     };
 
     friend class A2dpAudioStreamOut;