Merge pull request AgoraIO-Extensions#178 from AgoraIO/dev/fix

* fix android `VideoSizeChanged` bug

optimize

* fix iOS `FirstLocalVideoFrame` `VideoSizeChanged` `FirstRemoteVideo…

…Frame` `FirstRemoteVideoDecoded` bug

iOS fix

* fix `startPreview` `stopPreview` bug

* fix `toJson` bug

* add `startPreview` `stopPreview`

* remove `AudioSampleRateType.Type16000`

* fix Android `mapToChannelMediaInfo` crash

update readme

fix `getUserInfoByUid` bug

Dev/fix (AgoraIO-Extensions#134)

* fix AgoraLiveTranscodingUser.alpha type bug

* fix change log