3. Add Play Core library to your app - Play Core has been split into separate libraries

As the [doc for Play Core](https://developer.android.com/guide/playcore#include_playcore) linked in the third section shows, Play Core is now split into separate libraries. The codelab hasn't been updated yet.

Start with the third section of the codelab and compare its Play Core guidance with the linked Play Core documentation. Update the codelab to use the separate libraries described there; it is done when the instructions no longer rely on the obsolete combined Play Core library.

Written by the indexing model from the issue text.