Add Java annotation processors output to Android sources

apt.gradle

/**
 * Adds code generated by annotation processors to source folders.
 */

def aptOutputDir = project.file(new File(project.buildDir, "generated/source/apt"))
android.applicationVariants.all { def v ->
  def aptOutput = new File(aptOutputDir, v.dirName)
  v.addJavaSourceFoldersToModel(aptOutput)
  v.javaCompile.options.compilerArgs += [
      '-s', aptOutput.absolutePath
  ]
  v.javaCompile.doFirst {
    aptOutput.mkdirs()
  }
}