Improve transition handling during sign in

Change-Id: I60c521c89ad51b0dc80ddde507c74c35e40736a0

Many transition. Such slide. Much target. Wow.

* implement enter and exit transitions and more animations
* remove unused proguard references
* Introduce ViewUtils with a couple of Properties
* migrate Category display to RecyclerView
* replace GridView in category selection with RecyclerView
  * Espresso now requires espresso-contrib
  * enables use of notifyItemChanged
  * more control over updated items

Add backport scenario

* Available until API level 16
* Introduce Espresso tests
  * sign in flow
  * category display
  * quiz solving end to end
* Improve animation behavior
* Update README and screenshots

Demonstrate usage of Transitions API

Introduce custom windowSharedElementEnterTransition
to showcase usage of a custom transitionSet.

Improve listener situation.

Replaces "implements .*Listener" with
local variables where possible.