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.
Proposed edit:

If you feel adventurous, you may use the script [`/usr/local/bin/android-wait-for-emulator`](https://github.com/travis-ci/travis-cookbooks/blob/master/ci_environment/android-sdk/files/default/android-wait-for-emulator) and adapt your `.travis.yml` to make this emulator available for your tests. For example: