add comments for getVideoFormatPreference

Author: Xia Ning

Android/APIExample/lib-raw-data/src/main/cpp/io_agora_advancedvideo_rawdata_MediaPreProcessing.cpp

@@ -179,6 +179,9 @@ class AgoraVideoFrameObserver : public agora::media::IVideoFrameObserver
         return true;
     }
 
+    virtual VIDEO_FRAME_TYPE getVideoFormatPreference() override {
+        return FRAME_TYPE_YUV420; // Please don't modify videoFormatPreference in this raw data processing plugin, otherwise it won't work.
+    }
 };
 
 /**Listener to get audio frame*/
@@ -295,6 +298,8 @@ class AgoraAudioFrameObserver : public agora::media::IAudioFrameObserver
         writebackAudioFrame(audioFrame, _javaDirectPlayBufferMixAudio);
         return true;
     }
+
+
 };