Update Android Sample App ReadMe/Docs on where to put Credentials/Keys

[BorisWiggers]

While trying the Android SampleApps Ads out, it was not easy to determine which file to update my "Secret" and "AppKey".

The correct file to insert these credentials is "AdsViewActivity.java" where you can find,

    appKey = "UPDATE_APP_KEY_HERE";
    secret = "UPDATE_SECRET_KEY_HERE";