Adding "first packet received" notification to PeerConnectionObserver.

R=glaznev@webrtc.org, pthatcher@webrtc.org, tkchin@webrtc.org

Committed: https://crrev.com/42265a8cc3b3f3db4aa2c29005aed2fb4393adef
Cr-Commit-Position: refs/heads/master@{#11401}

Review URL: https://codereview.webrtc.org/1581693006 .

Cr-Commit-Position: refs/heads/master@{#11404}

webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java

@@ -1000,6 +1000,10 @@ public class PeerConnectionClient {
       // No need to do anything; AppRTC follows a pre-agreed-upon
       // signaling/negotiation protocol.
     }
+
+    @Override
+    public void onFirstMediaPacketReceived() {
+    }
   }
 
   // Implementation detail: handle offer creation/signaling and answer setting,