update_engine: add variable for DeviceQuickFixBuildToken policy

This CL makes DeviceQuickFixBuildToken policy visible in update_manager
which would be used in a follow up CL to skip applying
DeviceAutoUpdateTimeRestrictions when a quick fix build token is set.
This CL also adds quick fix token in UpdateCheckParams which is used
by UpdateAttempter to pass the token in OmahaRequestParams.

BUG=chromium:1117450
TEST=FEATURES=test emerge-${BOARD} update_engine

Change-Id: If43ad8cd2955bbeb1cbd1dcacac79742fa0a6a20
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2521189
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Tested-by: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/update_manager/chromeos_policy.cc b/update_manager/chromeos_policy.cc
index 262987e..24b8293 100644
--- a/update_manager/chromeos_policy.cc
+++ b/update_manager/chromeos_policy.cc
@@ -223,6 +223,7 @@
   result->rollback_allowed_milestones = -1;
   result->rollback_on_channel_downgrade = false;
   result->interactive = false;
+  result->quick_fix_build_token.clear();
 
   EnoughSlotsAbUpdatesPolicyImpl enough_slots_ab_updates_policy;
   EnterpriseDevicePolicyImpl enterprise_device_policy;