Fix non-critical updates on boards without an OOBE flow.

A recent change in the policy made update_engine to ignore available
updates if the OOBE flow is not completed and the update is not
critical. Nevertheless, some custom boards don't have a OOBE flow as
Chromebooks do and set is_oobe_enabled=false in the policy manager.
These board were not getting regular updates because the OOBE flow is
considered not completed in those cases.

This patch moves the is_oobe_enabled flag to the HardwareInterface class
together with the IsOOBEComplete() method and updates the callers to
check the IsOOBEEnabled() value before.

Bug: 28460247
Bug: 28553821
TEST=Added unittest for the disabled and not complete case.

Change-Id: Ifd3ac2dc5e7a43f6c24eb014b7e3eacad22e3ab3