Dispose audio source correctly in AppRTC Demo on Android.

Call dispose method on audio source in PeerConnectionClient dispose
method.

R=magjed@webrtc.org

Review URL: https://codereview.webrtc.org/2117463002 .

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Cr-Mirrored-Commit: 039083aedcd85b12f8fc2bb79b08b4500d29a4fb