Create wiki page for sdk gallery application

[jingtang10]

Create a wiki page for the sdc gallery application that includes the following topics

• what it is
• main functionalities
• code location
• APK location and sideload instructions
• local build instructions
• demo video

This page should be referenced from the wiki home page and the repo's README.md

[tunm1228]

I also don't know how to implement sdk in my project, there is no docs for calling sdk functions. You did it, please show me!