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/policy.h b/update_manager/policy.h
index 595fb7c..fb4a129 100644
--- a/update_manager/policy.h
+++ b/update_manager/policy.h
@@ -66,6 +66,9 @@
   std::string target_channel;
   // Specifies if the channel hint, e.g. LTS (Long Term Support) updates.
   std::string lts_tag;
+  // Specifies a token which maps to a Chrome OS Quick Fix Build, if imposed by
+  // policy; otherwise, an empty string.
+  std::string quick_fix_build_token;
 
   // Whether the allowed update is interactive (user-initiated) or periodic.
   bool interactive{false};