Refine back navigation search method.

- Remove embedded window check, EmbeddedWindowController#getByFocusToken
  should search with input token instead of window token, so it always
  return null nowaday.
- Skip search if the closing target isn't visibleRequested, usually that
  means the closing transition is collecting or playing, either case the
  window shouldn't receive another back invoked event. Inject the back
  key then next focus app will handle it.
- Use TYPE_CALLBACK if we cannot find any task below current task, this
  could also happen on some specific device.(e.g. No home)

Bug: 303266152
Bug: 309683765
Test: atest BackNavigationControllerTests
Change-Id: Ia1f5ec664ea7154d9c8ac59cbc247a51226dfdf1