The android build tools version used by all android builds is now centralised in one location This is so you don't have to upgrade the text in a hundred build.gradle files everytime a new version comes out. Refer to the variable project.rosandroid.buildToolsVersion in the following file:
project.rosandroid.buildToolsVersion = "19.1.0"
Quite often this will lag behind the android studio downloaded version. In most cases you can still use the value that is used here with no problem. Just run the android manager and make sure you install the version we are currently using (this is mentioned on the android studio download instructions).
If you have a use case where you absolutely require the new tools, follow the instructions below to get it upgraded for everybody.
Send a pull request against RosAndroid.groovy on the appropriate release branch.
This pull request should modify the variable shown above. You'll usually be doing this in tandem with an android gradle plugin upgrade. Damon has done this previously via a pull request that illustrates exactly what needs to be touched.
Update the version on the android studio download page on the wiki defining requirements.