Update TFLite Task API (Text) dependency and minSdkVersion

[rmorbach]

Hi.

Please update the section 11 of Add on-device Text Classification to your app with TensorFlow Lite and Firebase - Android Codelab tutorial replacing the dependency org.tensorflow:tensorflow-lite-task-text:0.0.0-nightly by org.tensorflow:tensorflow-lite-task-text:0.1.0.

With this it is also necessary to update de minSdkVersion to 21.

Regards.