Fix Android gradle build script

[lxcid]

This is similar to #27 but use a cleaner way to achieve the same result.

As of 0.56, we RN proj start declaring a few constants that libraries can query about.

Some library uses this technique so I'm porting it over here:

• https://github.com/oblador/react-native-vector-icons/blob/master/android/build.gradle
• https://github.com/oney/react-native-webrtc/pull/494/files