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.
In the future, we might do this by default, and offer an option to opt-out this "run on head" behavior. (still to be discussed in next development iteration)