Update Android Guide (rough and dirty)

These changes correspond to travis-ci/travis-cookbooks#337
Bump to Android SDK 23.0.2

Related to travis-ci/travis-ci#2596
Improve the updates of the Android Guide

Kudos and credits to @BanzaiMan review. Thanks!
Android: recommend to specify all dependencies

This practice ensure stable build environment even if some preinstalled
components are removed from the Travis VM. Unfortunately, it will also
increase the total build time (until we find a solution to this
problem).

See discussion in travis-ci/travis-ci#2296 for full details about the
advantages and drawbacks to rely on the pre-installed components.
Merge pull request #91 from travis-ci/android-updates

Update Android Guide