use gradle.properties for managing github credentials

- uses `gradle.properties` to manage the github credentials
- allows to run build by `./gradlew rDFD -PgithubClientId=dummy -PgithubClientSecret=dummer`

This is to fix the broken CI build due to missing file `credentials.properties`. You'll have to rename your local file `credentials.properties` to `gradle.properties` and adjust the variable naming as outlined above. I recommend putting the created `gradle.properties` into the `android` folder to avoid overriding the main projects file of the same name. 

Note: To fix the build you'll have to adjust the CI commands to supply valid credentials via the `-P` flag.