Implement JNI and objc implementation for Ice Candidate Pair Change event surfacing

Bug: webrtc:10419
Change-Id: I18528bf2526e933568bf052de76a434f012161da
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/148320
Commit-Queue: Alex Drake <alexdrake@google.com>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Reviewed-by: Alex Glaznev <glaznev@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28838}

sdk/android/instrumentationtests/src/org/webrtc/PeerConnectionTest.java

@@ -143,6 +143,9 @@ public class PeerConnectionTest {
     @Override
     public void onIceCandidatesRemoved(IceCandidate[] candidates) {}
 
+    @Override
+    public void onSelectedCandidatePairChanged(CandidatePairChangeEvent event) {}
+
     // TODO(bugs.webrtc.org/8491): Remove NoSynchronizedMethodCheck suppression.
     @SuppressWarnings("NoSynchronizedMethodCheck")
     public synchronized void setExpectedResolution(int width, int height) {