Migrate TensorFlow Lite out of tensorflow/contrib

This change moves //tensorflow/contrib/lite to //tensorflow/lite in preparation
for TensorFlow 2.0's deprecation of contrib/. If you refer to TF Lite build
targets or headers, you will need to update them manually. If you use TF Lite
from the TensorFlow python package, "tf.contrib.lite" now points to "tf.lite".
Please update your imports as soon as possible.

For more details, see https://groups.google.com/a/tensorflow.org/forum/#!topic/tflite/iIIXOTOFvwQ

@angersson and @aselle are conducting this migration. Please contact them if
you have any further questions.

PiperOrigin-RevId: 219536476