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.
I agree that the flux of preinstalled components should be expressly noted. How about:

> While the following components are preinstalled, the exact list may change without prior notice. We recommend that you explicitly specify the required components for your project.