[spark][omnibox] Add an entry to chrome://flags.

Enabling this entry allows users to override any configuration set by experiment or policy.

This change also adds a check to see if the "SearchAggregatorProvider" Feature is enabled when deciding whether or not to run EnterpriseSearchAggregatorProvider or DocumentProvider. Previous to this change, if the Feature was disabled but the other configuration values were still provided, EnterpriseSearchAggregatorProvider would run but not do anything. After this change, if "SearchAggregatorProvider" is disabled, the DocumentProvider will be run instead. This allows users to manually disable the aggregator integration from chrome://flags.

Finally, this removes chrome://flags#enable-search-aggregator-policy. That feature is enabled by default and probably doesn't need to be set anymore.

Fixed: b:402501223
Change-Id: I6330887b69d91358e399ea3b3c87a600a3923095
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6345213
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431495}