AudioPolicyService: Synchronize access to AudioPolicyManager

Synchronize access to APM when getDevicesForStream is called
on APM.

Change-Id: I2ba6922341f035375270b02000ef5a7e078f6b5a
diff --git a/services/audiopolicy/service/AudioPolicyInterfaceImpl.cpp b/services/audiopolicy/service/AudioPolicyInterfaceImpl.cpp
index 793c26a..58ecb11 100644
--- a/services/audiopolicy/service/AudioPolicyInterfaceImpl.cpp
+++ b/services/audiopolicy/service/AudioPolicyInterfaceImpl.cpp
@@ -459,6 +459,7 @@
     if (mAudioPolicyManager == NULL) {
         return AUDIO_DEVICE_NONE;
     }
+    Mutex::Autolock _l(mLock);
     return mAudioPolicyManager->getDevicesForStream(stream);
 }