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.
Perhaps we can clarify where this command is expected to be executed. Running it on the worker is more definitive, but takes an extra build to get them; running it on a dev machine is quicker, but may not be accurate.