Clear Shill stub config in offline file manager tests

The Shill stub client fakes ethernet and wifi connections during
testing. Clear its config during offline tests to simulate a lack of
network connectivity.

As a side effect, fileManagerPrivate.getDriveConnectionState will no
longer need to be stubbed out, as it will now think the device is
offline and return the appropriate result.

Bug: 925272
Change-Id: Idd6cb44325cfde4991d3b1e64185a28e8655c733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578149
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654782}