Use Gradle api to generate Javadocs instead of Android plugin one

* Refactor Java docs task so it uses `variant.javaCompile.source` instead of `android.sourceSets.main.allJava` because the second one will not longer be available in Android Gradle plugin 0.11 
* Latest build tools
* Keep android plugin 0.10 because 0.11 seems to have issues with Roboelectric