Make sure pages are always accessible via left/right arrow keys.

- Handle NextPageFirstItem as first focusable item in reading order
- Handle PreviousPageLastItem as last focusable item in reading order
- Check the hotseat after the workspace in both cases above
- Dpad horizontal navigation (left/right) uses these as a last
  resort (Rule3) to guarantee an item takes focus if a page exists

Note that it is necessary to search for a focusable item because
widgets are not yet focusable.

Bug: 25591057
Change-Id: I953648bd76c657d660a38427fdd4108bf9963c23