Bug 1678765 - Fix broken "Search with" action text for heuristic results that are restyled to look like search results. r=harry

This was a little harder than it seemed like it should be because we show/hide
the title separator in two places, in `UrlbarView.updateRow` and in the CSS.
This patch gets rid of the show/hide in `updateRow`, so now we show/hide only in
the CSS. The decision to show/hide in `updateRow` was based on whether the
result has action text (`actionSetter`) or is a URL (`setURL`). We already had a
`has-url` attribute that corresponds directly to `setURL`, so adding a similar
`has-action` attribute that corresponds to `actionSetter` lets us show/hide only
in the CSS.

The second part of this patch is to actually fix the bug. For that, the existing
`show-action-text` attribute does what we want in the CSS; the only problem is
that we currently set it only when there's no selected row, but we need to also
set it when there's no selected row but a one-off is selected. We have a similar
block -- the one with a comment about restyling the heuristic to look like a
search result -- so I removed the block that currently sets/removes the
attribute and moved the set/removal there. I also renamed the attribute
`restyled-search` to better semantically describe what's happening, but if you
disagree I can restore the old name.

Depends on D97843

Differential Revision: https://phabricator.services.mozilla.com/D98429