Create the Java Wrapper of RtpReceiverObserverInterface.

Create the RtpReceiver.Observer which is a Java wrapper over the webrtc::RtpReceiverObserverInterface.
The callback function onFirstPacketReceived will be called whenever the first audio or video packet it received.

BUG=webrtc:6742

Review-Url: https://codereview.webrtc.org/2531333003
Cr-Commit-Position: refs/heads/master@{#15464}
diff --git a/webrtc/api/rtpreceiver.cc b/webrtc/api/rtpreceiver.cc
index 02e1650..80a0256 100644
--- a/webrtc/api/rtpreceiver.cc
+++ b/webrtc/api/rtpreceiver.cc
@@ -114,7 +114,7 @@
 void AudioRtpReceiver::SetObserver(RtpReceiverObserverInterface* observer) {
   observer_ = observer;
   // Deliver any notifications the observer may have missed by being set late.
-  if (received_first_packet_) {
+  if (received_first_packet_ && observer_) {
     observer_->OnFirstPacketReceived(media_type());
   }
 }
@@ -212,7 +212,7 @@
 void VideoRtpReceiver::SetObserver(RtpReceiverObserverInterface* observer) {
   observer_ = observer;
   // Deliver any notifications the observer may have missed by being set late.
-  if (received_first_packet_) {
+  if (received_first_packet_ && observer_) {
     observer_->OnFirstPacketReceived(media_type());
   }
 }