Skip non-critical updates during OOBE update check

If the system has not yet completed OOBE, and Omaha returns a response
with an empty deadline field, avoid initiating a background update
because doing so may violate an enterprise policy that gets pushed
down post-enrollment.

Bug: None
BUG=chromium:587101
TEST=`FEATURES=test emerge-link chromeos-base/update_engine`
TEST=powerwash, install lsb-release file from an old stable build,
     repeat OOBE, verify that download is skipped via logs, then verify
     that update proceeds after logging into a standard gmail account

Change-Id: I6e6fae02c609b2763abc6e9960bf6d87dd3534e2