cros: Add histogram for tracking weekly active desks.

This CL adds a histogram for tracking weekly active desks. Desks now
store whether they've been interacted with and the Desk class has a
global variable which counts the number of weekly active desks. These
metrics are persisted across sessions and stored in user prefs.

Test: manual + added
Bug: 1133391
Change-Id: Ia8286b598aa43887a68887bc6861ae7054940623
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2783120
Commit-Queue: Jeremy Chinsen <chinsenj@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#870126}