Exercise 4 TODO 6 and 8 registerIdlingResources() and unregisterIdlingResources() are deprecated

[mkillewald]

https://stackoverflow.com/questions/47078005/why-is-espressos-registeridlingresources-deprecated-and-what-replaces-it

In TODO (6) Instead of:
Espresso.registerIdlingResources(mIdlingResource);
use:
IdlingRegistry.getInstance().register(mIdlingResource);

In TODO (8) instead of:
Espresso.unregisterIdlingResources(mIdlingResource);
use:
IdlingRegistry.getInstance().unregister(mIdlingResource);