audio policy: cleanup AudioPolicyService destructor

No need to clear strong pointers in the destuctor.
Add explicit exit() command to mOutputCommandThread.

Change-Id: I60bb248fe6a20bfd8a55358ef139692b93df0238
diff --git a/services/audiopolicy/AudioPolicyService.cpp b/services/audiopolicy/AudioPolicyService.cpp
index 918c25c..2811475 100644
--- a/services/audiopolicy/AudioPolicyService.cpp
+++ b/services/audiopolicy/AudioPolicyService.cpp
@@ -122,10 +122,8 @@
 AudioPolicyService::~AudioPolicyService()
 {
     mTonePlaybackThread->exit();
-    mTonePlaybackThread.clear();
     mAudioCommandThread->exit();
-    mAudioCommandThread.clear();
-
+    mOutputCommandThread->exit();
 
     // release audio pre processing resources
     for (size_t i = 0; i < mInputSources.size(); i++) {