Search preservation does not work with duckduckgo in safest security level

Steps to reproduce:

1. Make sure "Show search terms in the address bar on results pages" is checked in search settings.
2. Set the browser to use the safest security level.
3. Start a search in the urlbar with duckduckgo.

Unlike when using the lower security levels, the search term is not saved in the urlbar. Instead, the user sees the search url html.duckduckgo.com/html?q=. I suspect that the underlying cause is that the duckduckgo metadata search url does not match the url used in the safest mode. We intervene in the search code to switch it from duckduckgo.com to html.duckduckgo.com.

This isn't necessarily an issue. On the one hand, it makes it clear to the user that a different search engine url is used when in the safest security level. But it is also inconsistent behaviour.

@donuts and @felicia what do you think?

/cc @pierov