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This adds a row in the Nearby Settings subpage for the "Data usage"
setting, as well as the dialog which appears when the user clicks the
edit button on the row. The dialog saves the selected option to prefs,
but in a future CL this will be updated to use a mojo.

Screenshot of the row in the subpage:
  https://screenshot.googleplex.com/5BGciifsy8m
Screenshot of the dialog:
  https://screenshot.googleplex.com/2cVEQtz9yVx

Specs in Carbon: https://goto.google.com/orazy

Bug: b:154866240
Change-Id: I25749a71a3b78ebf4b75350a60ab2ede744b3927
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311050
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794276}