AudioService: BtHelper: Add missing GuardedBy comment

Add missing comment to synchronized method
BtHelper.stopAndRemoveClient() add by commit 7647273f.

Test: make
Change-Id: Ie80b1f8ed0ccfeb6017ec4b55f82ccb2af51574d
diff --git a/services/core/java/com/android/server/audio/BtHelper.java b/services/core/java/com/android/server/audio/BtHelper.java
index ded0f9a3..8de31d9 100644
--- a/services/core/java/com/android/server/audio/BtHelper.java
+++ b/services/core/java/com/android/server/audio/BtHelper.java
@@ -450,6 +450,7 @@
     }
 
     @GuardedBy("AudioDeviceBroker.mDeviceStateLock")
+    // @GuardedBy("BtHelper.this")
     private void stopAndRemoveClient(ScoClient client, @NonNull String eventSource) {
         AudioService.sDeviceLogger.log(new AudioEventLogger.StringEvent(eventSource));
         client.requestScoState(BluetoothHeadset.STATE_AUDIO_DISCONNECTED,