ObjC: Notify local video track

The macOS demo add camera preview in didReceiveLocalVideoTrack callback, but this callback is never called.

Bug: webrtc:9276
Change-Id: I60b9cc69672f3654d4e36de0e8140e0bbb957540
Reviewed-on: https://webrtc-review.googlesource.com/77950
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23458}
diff --git a/AUTHORS b/AUTHORS
index 83a74af..15f2589 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -62,6 +62,7 @@
 Todd Wong <todd.wong.ndq@gmail.com>
 Maxim Pavlov <pavllovmax@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
+Piasy Xu <xz4215@gmail.com>
 
 &yet LLC <*@andyet.com>
 Agora IO <*@agora.io>