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This CL updates TetherService and TetherHostFetcher to use the
MultiDeviceSetupClient's GetFeatureState() function to determine whether
the feature should be active or not. This fixes a few edge cases:
  (1) If the Instant Tethering user preference is enabled, but the
      Better Together suite is disabled, we should not start up the
      feature.
  (2) If the preference is enabled, but the host device does not support
      Instant Tethering, we should not start up the feature.

This CL eliminates the MULTIDEVICE_HOST_UNVERIFIED metrics enum since
an unverified host results in NO_AVAILABLE_HOSTS; it replaces that enum
value with BETTER_TOGETHER_SUITE_DISABLED, which can occur in practice.
Note that because the old metric value has not yet been pushed to users,
it is okay to replace it with a new value instead of deprecating it.

Bug: 875502, 824568
Change-Id: Ia85f0ba31871c94ed9eb0cb2ef3c4699bb0000dd
Reviewed-on: https://chromium-review.googlesource.com/1196172Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587894}