Android SDK: pre-install build-tools 21.1.1

I removed former versions 20.0.0 (and transient 21.0.2).
As agreed in https://github.com/travis-ci/travis-cookbooks/pull/360,
we only pre-install the "latest" build-tools version
Android SDK: remove inexistent 'android-support'

Revert 9397bd1f3236ab5081b5edfb474239cd25f400ce.
The extra-android-support component was already part of preinstallation
list.
Android SDK: minor cleanup

Remove a forgotten TODO that initially cames via
https://github.com/travis-ci/travis-cookbooks/pull/337
and d6773f9aa29cc29c8ebdb94bfeb4fc9bf1fa992a
Update changelog (Android SDK components updated)

[ci skip]