update the demo gifs in readme.md

[shivamk01]

We have two gifs in the invisible watermark section, with the code comes to the newer version, the demo gifs have been out of date, I think we can update it using the newest version of demo.

All you need is to:

Fork this repo and compile it in your own machine. Run the latest demo apk in a real android device or an emulator. Record the screen using adb, and convert it to two gif pictures. Update the gifs in README.md and README-CN.md. Note: The gifs cannot be too large or it will take a really long time to load.