Hid fb secrets in local.properties

Fixes #339 (closed)

Changes :
1)Now,from app level build.gradle,it will first fetch api key and api scheme from root level local.properties then it will be passed to manifest file as a string which will then be used in our app.
2)updated gitlab ci
3)updated contributing.md

Appetize Link :
https://appetize.io/app/jkhn57ejunr06ypj2phpdca1m0?device=pixel4xl&scale=75&orientation=portrait&osVersion=10.0