GH-2213 bugfix: Broken Next button

[wlycdgr]

Fixes bug where Next button would break if the internal name of the user's chosen search returned by browser.search.get() did not match any of our search engine name constants.

I added some protection to reduce the risk of unexpected values breaking our code, and added a new dawn_setup_number to use for cases when we don't recognize the chosen search.

Finally, I made a best guess for what the Search Encrypt internal name will turn out to be to improve the chance that we won't have to make a code change for this. If we start getting a lot of "unrecognized" pings once that engine is added on the Dawn side, we'll want to look here first.

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[benstrumeyer]

LGTM, approved