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Metrics: Refactor FamilyUserDeviceMetricsTest to use LoggedInUserMixin

This CL
* Refactors FamilyUserDeviceMetricsTest to use LoggedInUserMixin,
eliminating a lot of duplicated code that's already encapsulated in
LoggedInUserMixin and shortening the browser test class.
* Breaks monolithic tests into smaller, modularized pieces to make
them easier to understand and debug when they break.
* Removes some unneeded includes, like UserActionTester and
ScopedTestingCrosSettings.

Bug: 1133856
Change-Id: I098a3028dd021379a46fa619e7f597717507e1ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602701
Commit-Queue: Toby Huang <tobyhuang@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846350}