Initial commit for confirm device credentials sample based on the EAP sample.

Changed:
- Conform to the DevRel template
- Handle the case when the user hasn't set up a lock screen
  to fall back to showing a message instead of letting the app crash
  - Some minor refactoring

  Bug: 20035162

  TODO Add InstrumentationTests once it can be launched.
  At this moment it can't be launched saying different API
  (E.g. Skipping device 'Nexus 5 - MNC', due to different API preview 'MNC' and
  'android-MNC')

  Taken over from this CL ag/682639, since it turns out now
  mnc-preview-docs is the right branch for M preview samples.

Change-Id: Idfb55a07c22e793179ad0514617c9d1e6c5a51b6