[css-nav-1] Fix the unreachability for a focusable element inside the search origin

[jihyerish]

Consider the focusable element which fully overlaps with another focusable element.

Detailed changes are as below:

1. Add another condition to the definition of 'inside area'

   • To consider the focusable element which fully overlaps with search origin as a candidate for enabling the focus to move there.

2. Make the difference the way to select 'insider' depending on whether the search origin is a spatial navigation container or not

   • If the search origin is a container, consider the partially visible focusables as the insider.
   • Else (if the search origin is the general element), consider only the fully overlapped focusables as the insider.
     • NOTE: The partially overlapped focusable isn't included in the set of insider.

3. Modify the way to select candidates among visible focusable elements

   • To measure the distance from the elements which is outside to the search origin.

Close #3386