Surface contextual search ad actions; enter and keep @page keyword mode

This CL does several things to make the omnibox work better with the
contextual search provider (CSP):
- The CSP produces pedal-like action matches to advertise contextual
  search when *not* in '@page' mode, only for empty input/zero suggest.
  For now these are high relevance to make the list, but eventually
  need to be sorted to the bottom of the list instead of top.
- The ContextualSearchAction is renamed to
  ContextualSearchFulfillmentAction to avoid ambiguity with the ads.
- The "Ask about this page" action puts the omnibox into '@page'
  keyword mode for direct query entry with page-contextual suggestions.
- The "Search with Google Lens" action invokes the lens overlay
  so the user can select a region for lens search.
- The CSP uses placeholder matches to keep the omnibox in keyword
  mode. This is currently required due to how the omnibox deals
  with keywords and default matches.
- The OmniboxEditModel is slightly refactored to support more general
  keyword mode entry, as it only supported default search engine
  entry before.
- Invocation of lens overlay on '@page' keyword mode entry is
  removed because it's too soon, not intentional enough. Using
  an action match is now the only way to invoke lens, though we may
  want to add more indirect support via '@page' starter pack later.
- Matches from suggest are modified by the provider to preserve
  keyword mode (and there's a TODO with details about how to clean
  this up in next steps).
- Eliminates surfacing the '@page' starter pack keyword match from
  the FeaturedSearchProvider; it wasn't sufficient or a right fit.
- Simplifies contextual search fulfillment action attachment.

Note that the behavior with this CL may not work as expected, and
this is intentional to show the gaps and what remains to be done.
In particular, clicking "Ask about this page" will enter the scope
but receive no suggest results because there's no background lens
invocation yet. If you then invoke the "Search with Google Lens"
action, lens appears for the current page, and suggestions from
omnibox can work. Then go back and invoke the "Ask..." action again,
and suggestions will appear because lens inputs are populated.

Bug: 402771688, 400812940, 400952597
Change-Id: Idf144fc7dff51c57853385e81383ebbbe8548dea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6384376
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Nihar Majmudar <niharm@google.com>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437189}