Obfuscate library project

对library project的混淆需要在主工程的混淆配置文件中设置
参考：https://stackoverflow.com/questions/10982344/is-proguard-cfg-needed-for-library-projects#10992604
https://stackoverflow.com/questions/8088958/how-to-obfuscate-an-android-library-jar-file-using-proguard-in-eclipse
https://www.guardsquare.com/en/proguard/manual/introduction