Update Android Guide (rough and dirty)

These changes correspond to travis-ci/travis-cookbooks#337

@BanzaiMan @joshk Please review and improve this draft. To be merged when the new VM image is available.

Following issues travis-ci/travis-ci#2279, travis-ci/travis-ci#2466, travis-ci/travis-ci#2470 and travis-ci/travis-ci#2476 can be closed when this new VM will be available.
Yep, the point here is to identify if we should provide some `.travis.yml` keys for these steps. So far, it is not clear how "unique" is the way to do. Better to wait for user requests, so I agree to warn a bit more.