There seem to be a couple of ways to skin this cat but one way would be:

Example adding TAG DPF

In build.gradle the module not the project

buildTypes {
        release {
            ...
            buildConfigField "String", "TAG","\"DPF"\""
        }
        debug {
            ...
            buildConfigField "String", "TAG", "\"DPF\""
        }
    }

• Then rebuild the project. Check the generated BuildConfig in the Java (generated) folder.
• You should see something like public static final String TAG = "DPF";
• Now use BuildConfig.TAG wherever you want to use the constant