🧇 Disambiguate remaining Fallback vs Default template URL getters

Clarifies in DefaultSearchManager and TemplateURLPrepopulateData
APIs that:
- "Default" refers to the default search engine, that is used for
  omnibox searches
- "Fallback" refers to the search engine that can be used when
  there isn't one selected by another mean, and picked from the
  prepopulated set.

As part of it, we remove the optional argument that allowed to get
the index of Google when calling  GetPrepopulatedEngines(). Instead,
we can call the preexisting (but renamed)
GetPrepopulatedFallbackSearch() function to obtain it.

Bug: 330544981
Change-Id: I2afe6e90d440160e6ec78be1ec08bb981311d243
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5584838


Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Jack Yammine <jyammine@google.com>
Commit-Queue: Angela Yoeurng <yoangela@chromium.org>
Auto-Submit: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313068}