Check missing build-tools

The android SDK manager, under same circumstances (mainly after an update), can leave you with no build tools. The "build-tools" directory would be empty, and getBuildTools() will return null. This results in a NPE, which is difficult to diagnose (see [Issue 397](https://code.google.com/p/maven-android-plugin/issues/detail?id=397) and [Issue 432](https://code.google.com/p/maven-android-plugin/issues/detail?id=432) on google code).
This pull request checks for this case and produce a more meaningful InvalidSdkException.