Consistent naming and proto best practices for DeviceRebootOnUserSignout

Follow proto best practices by having an _UNSPECIFIED enum value for
DeviceRebootOnSignoutMode.
Make naming of DeviceRebootOnUserSignout policy / protos consistent.

Bug: b:77799573
Change-Id: I01b0bfa1ea4eff0c52d13953d9d865793f2b089f
Reviewed-on: https://chromium-review.googlesource.com/c/1475464
Reviewed-by: Igor <igorcov@chromium.org>
Reviewed-by: Bartosz Fabianowski <bartfab@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633063}
9 files changed