Review Mozilla 1916074: Query user whether they want to install engine when using contextual search

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1916074

Description

See install path 3 in #44117 for more info.

After you use a temporary search engine you will get a notification asking if you want to add the search engine permanently. I think this might be limited to non-private browsing.

#44117 covers the long term solution. This issue should just be focused on whether we want this disabled for the 15.0 release.

/cc @donuts

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