Android Studio Javadoc: Cannot find symbol

You should also add all your dependency to the javadoc.classpath. Try this:

task javadoc(type: Javadoc) {
    source = android.sourceSets.main.java.srcDirs
    classpath += project.files(android.getBootClasspath().join(File.pathSeparator))
}

afterEvaluate {
    javadoc.classpath += files(android.libraryVariants.collect { variant ->
        variant.javaCompileProvider.get().classpath.files
    })
}