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This adds the first of our new search providers for the files
ranking project, which provides zero-state local files suggestions
to the CrOS launcher. The design doc can be found here:

go/cros-zero-state-files-ranker

A quick overview:

 - The ZeroStateFileProvider is a new search provider that tracks
   recently-used files to display in zero state.

 - It does this by feeding notifictions from the FileTasksObserver
   into a RecurrenceRanker (Dolphin) model.

 - It's unrelated to the LauncherSearchProvider, which provides file
   results for search queries. The LSP only provides results given a
   query, and the ZSFP only provides results without one, so these
   are mutually exclusive. Note this has been implemented separately
   from the LSP so as not to further tie the launcher to the Files
   app API and javascript.

This CL has some TODOs related to parallel CLs: the search result
icon needs to be set, DriveFS results need to be filtered out, and
UMA metrics added.

The use of the new provider is gated behind a disabled-by-default
feature flag. We'll enable this once the rest of the files ranking
code has been merged.

Bug: 959679
Change-Id: I7b9697906cf13793e34fc79097b379cbe8becb48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755623Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688643}