Set the audio stream for phone calls

This tells the system which stream to adjust volume for when the
session is active.

Change-Id: Ib594b70c53bb432a4e8300434006729b483078d5
diff --git a/src/com/android/telecomm/HeadsetMediaButton.java b/src/com/android/telecomm/HeadsetMediaButton.java
index db34f59..6ca9b86 100644
--- a/src/com/android/telecomm/HeadsetMediaButton.java
+++ b/src/com/android/telecomm/HeadsetMediaButton.java
@@ -114,6 +114,7 @@
         mSession.addCallback(mSessionCallback);
         mSession.setFlags(MediaSession.FLAG_EXCLUSIVE_GLOBAL_PRIORITY
                 | MediaSession.FLAG_HANDLES_MEDIA_BUTTONS);
+        mSession.setPlaybackToLocal(AudioManager.STREAM_VOICE_CALL);
     }
 
     /**