Only show the site search results separator when input is focused

[owenatgov]

Something we missed in #4220. We control how the separator is displayed via 2 accessible autocompelte states:

1. the input has aria-expanded="true", indicating it has results
2. the input has aria-expanded="false" but also no aria-describedby attribute, indicating that the 'no results' dialog is visible

The 2nd state however is also true when a user has entered text into the input and clicked away, resulting in the border being visible when there's no dropdown:

This change uses app-site-search__input--focused as an additional hook for that state. We already do something similar for controlling the focus state of the input further down.

[netlify[bot]]

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