Fix androidJavadocs

`androidJavadocs` currently fails with the following error and generates an empty javadoc.jar:

`Converting class com.android.build.gradle.internal.api.DefaultAndroidSourceDirectorySet to File using toString() method has been deprecated and is scheduled to be removed in Gradle 2.0. Please use java.io.File, java.lang.String, java.net.URL, or java.net.URI instead.`

This PR fixes this error and generates a correct javadoc.jar.