Update device_second_factor_authentication device policy enum

As requested, add an 'Unset' value for the enum controlling the mode of
the integrated second factor authentication feature rather than using
the 'Disabled' state.

Bug: b/35545754
Change-Id: I3438eff45ba5740660cd313e0375d994a2a6d391
Reviewed-on: https://chromium-review.googlesource.com/548035
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Thiemo Nagel <tnagel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#482239}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 57d9479f0ba28f1635d80c82c61d7d8582ec8fcd