Create proper AAR for android bundling

[tyvsmith]

With the project moved to Gradle, it should now be straight forward to build first class Android integration by creating an AAR as one of the outputs.

[isaacbrodsky]

The publish script is able to handle multiple publications, so #184 is now released as com.uber:h3-android. I've updated my sample app at https://github.com/isaacbrodsky/h3-android-sample to reflect that. Thanks @gpolak !