Move special Configuration.java properties into gradle-generated BuildConfig

[ncalexan]

I see that the gradle way of including build-time configuration is using BuildConfig. For a simple (documented!) example, see https://github.com/mozilla/MozStumbler/. They include API keys just like you want in gradle.properties and then generate BuildConfig.java from those properties. This project should follow suit; flag me for review.

[EricEdens]

The current milestone for landing in M-C doesn't use any external APIs; so I'll close this for now, but we'll want to address this when we do add third-party APIs