Make WebRTCAudioTrack class public.

To access its public API.

TBR=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/2042523002 .

Cr-Commit-Position: refs/heads/master@{#13044}

webrtc/modules/audio_device/android/java/src/org/webrtc/voiceengine/WebRtcAudioTrack.java

@@ -22,7 +22,7 @@ import android.os.Process;
 
 import org.webrtc.Logging;
 
-class WebRtcAudioTrack {
+public class WebRtcAudioTrack {
   private static final boolean DEBUG = false;
 
   private static final String TAG = "WebRtcAudioTrack";