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Before this CL, the multi-device settings displayed Instant Tethering
settings using exactly the same mechanism used by the network settings.
However, this is undesirable because (1) it does not display the
"prohibited" icon when a device administrator prohibits the feature and
(2) it does not display the correct text as a label for the item.

This CL changes the settings subpage to use the same infrastructure as
the rest of the features. As a result, this also requires that
TetherService be updated; previously, TetherService was responsible for
changing the "enabled" user pref itself, but now it needs to support that
mode as well as responding to changes of the "enabled" pref.

Bug: 896324, 884830
Change-Id: I083a8e8e07725130058db35ee540dd6ad733d421
Reviewed-on: https://chromium-review.googlesource.com/c/1297594Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602199}