Second pass for Espresso-powered ViewPager tests.

* Rename ViewPagerActions to ViewPagerUtils and add two matchers.
* Move the adapter class from activity to the test and extract
common functionality to a base class.
* Have two specific adapter implementations for testing adapter
switch at runtime.
* Add a test method to test for ViewPager page content.
* Add another test method to test for ViewPager page content after
setting a new adapter.

Change-Id: I6f39b53673bbf2e10dd14d9bd749154edef85215
6 files changed