The Android / iOS application specified in the console refers to the final package you intend to ship. So if you're building a project using Unreal for Android then you would add an Android project in the console.
FYI: Using the Firebase C++ SDK with Unreal is a little tricky at the moment for Android as you'll need to include the Java AARs for Android builds
https://firebase.google.com/docs/cpp/setup#dependencies_1 . Last time I looked, Unreal wasn't using Gradle to package Android applications which makes this process relatively tricky. If you're familiar with Unity you could try to copy what our Jar Resolve Unity Plug-in does
https://github.com/googlesamples/unity-jar-resolver and build something similar for Unreal. Or even better would be to patch Unreal to support AAR dependencies by switching the packaging step to use Gradle.