Travis CI: add new license key, SDK 26 and build tools 28.0.3.
Travis CI: add new license key, SDK 26 and build tools 28.0.3.

- Drop unused tools, extra repo and platform-tools.
- Use pre-installed JDK.
- Print Gradle version.
- Only assemble debug build (does not run lint).

greenrobot/android-database-performance,greenrobot/android-database-performance