Added sanity UI tests in the sample app

[zawadz88]

This PR:

• Adds Support test library 1.0.0 with Test Orchestrator and Espresso 3.0.0
• Updates Android Gradle Plugin to 3.0.0-beta2
• Performs sanity UI tests in the sample app by making screenshots of sample screens

This solves issue #189

See:
https://developer.android.com/training/testing/junit-runner.html
https://developer.android.com/studio/build/gradle-plugin-3-0-0.html

[codecov-io]

Codecov Report

Merging #190 into develop will decrease coverage by 0.43%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #190      +/-   ##
=============================================
- Coverage      66.14%   65.71%   -0.44%     
- Complexity       198      202       +4     
=============================================
  Files             29       30       +1     
  Lines            963      980      +17     
  Branches          85       85              
=============================================
+ Hits             637      644       +7     
- Misses           281      290       +9     
- Partials          45       46       +1

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/stepstone/stepper/StepperLayout.java	41.3% <ø> (ø)	40 <0> (ø)	⬇️
...stepper/internal/widget/RightNavigationButton.java	41.17% <0%> (ø)	4% <0%> (?)

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[zawadz88]

Code coverage seems smaller due to the fact that previous code coverage excluded some files which it shouldn't. This was fixed now.