Add test for LinkAccessibilityHelper

- For the AccessibilityDelegateCompat methods, add tests that the
  delegation is done correctly.
- Refactored LinkAccessibilityHelper to make the dependency direction
  clearer.

Test: ./gradlew connectedAndroidTest test
Change-Id: I6132c0820ee6de1b9cc71a2838bdf05a34d7d2af