[DocsUI] Enhance UI test debugging

* Add log to indicate which layout (fixed/rail/drawer) the test is
  running with.
* Support screenshot and view hierarchy for test failures.
* Support screen recording (regardless of test succeed or failure).

The screenshot and view hierarchy are implemented by the custom test
runner called "Functional".
* This Functional test runner supports capture screenshot and view
  hierarchy right after test failure, ScreenshotOnFailureCollector
  and custom Rule only support capturing these after the "@After"
  (tear down) which is not what we want.
* In order to use this runner, UI tests need to:
  * either extend from the base class ActivityTestJunit4 (which has the
    RunWith annotation),
  * or annotate the test class with@RunWith(Functional::class).

The screen record is implemented by ScreenRecordCollector, check
CL:34163023 for more details.

Bug: 426030627
Test: EXEMPT testing code
Flag: EXEMPT testing code
Change-Id: Ieed6f801b96b1e7495dbde56e86344dfba0ec551