Surface the standardized ICE connection state to mobile clients.

This CL adds the callback on changes of the ICE connection state
following the standardized transitions
(https://www.w3.org/TR/webrtc/#dom-rtciceconnectionstate) to the
Android and the iOS SDKs.

Bug: None
Change-Id: I6133391fa54dd4e09016f29dddb85e4a0e270878
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138181
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Amit Hilbuch <amithi@webrtc.org>
Reviewed-by: Alex Glaznev <glaznev@webrtc.org>
Commit-Queue: Qingsi Wang <qingsi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28127}

sdk/android/api/org/webrtc/PeerConnection.java

@@ -98,6 +98,10 @@ public class PeerConnection {
     /** Triggered when the IceConnectionState changes. */
     @CalledByNative("Observer") void onIceConnectionChange(IceConnectionState newState);
 
+    /* Triggered when the standard-compliant state transition of IceConnectionState happens. */
+    @CalledByNative("Observer")
+    default void onStandardizedIceConnectionChange(IceConnectionState newState) {}
+
     /** Triggered when the PeerConnectionState changes. */
     @CalledByNative("Observer")
     default void onConnectionChange(PeerConnectionState newState) {}