Report PreviewsEligibilityReason as offline when device is offline

Currently, if the device is offline, PreviewsEligibilityReason
records the reason for not showing previews as network quality
unavailable.

This CL changes it to record the reason as device offline.
Network quality unavailable reason is now used for only the cases where
NQE (network quality estimator) reports ECT as Unknown.

Bug: 912725
Change-Id: I0af0977770e2123f6ccd0317f8ebbfaed1aac9b2
Reviewed-on: https://chromium-review.googlesource.com/c/1366835
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614621}
5 files changed