docs: Refactor testing docs to remove/update obsolete guidance:

  * Added new 'Getting Started with Testing' training to contain common
      information on how to configure and run tests with Android Studio.
  * Removed the obsolete 'Testing Your Activity' training.
  * Deleted obsolete testing content under 'Tools > Workflow'.
  * Moved accessibility testing checklist to 'Implementing accessibility'
      training section.
  * Moved content provider and service testing topics into a new
     'Testing App Integrations' training.
  * Updated the service testing training to reflect our recommended
approach of using ServiceTestRule, instead of ServiceTestCase.
  * Renamed 'Testing from Other IDEs' topic to
     'Testing from the Command Line' and moved it under 'Testing Tools'.
  * Revised 'Testing Fundamentals' topic and moved it under 'Testing Tools'.
  * Updated the landing page fo 'Best Practises for Testing' to provide a
     central location for testing-related resources.
bug: 20722624

Change-Id: Ic1cbc79b48916dcae1c09bf5114ba8aef13d6f27