update_engine: Use a second ActionProcessor for UpdateBootFlagsAction

The crrev.com/c/2519843 mistenly merged an ActionProcessor from
RealSystemState to the one in UpdateAttempter. This is problematic
because these two ActionProcessors used to run at different times (and
sometimes simultanously) and on different objects. But after they were
merge, they are colliding. For example UpdateAttempter::StartUpdater()
is adding Actions that run 45 seconds later, but the
UpdateAttempter::Update() starts the same ActionProcessor which is a
bug.

BUG=b:173251868
TEST=emerge and cros deployed, then run:
test_that  --args="job_repo_url=http://100.115.168.195:8082/static/reef-release/R88-13591.0.0/autotest/packages" chromeos6-row3-rack10-host7.cros autoupdate_ForcedOOBEUpdate.full

Change-Id: I7716e9ae2e77bbc4caff5273cd3fbc22e1ed7f5b
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2542962
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/cros/update_attempter.h b/cros/update_attempter.h
index 3678707..aeb7f9b 100644
--- a/cros/update_attempter.h
+++ b/cros/update_attempter.h
@@ -461,6 +461,8 @@
 
   std::unique_ptr<ActionProcessor> processor_;
 
+  ActionProcessor aux_processor_;
+
   // Pointer to the certificate checker instance to use.
   CertificateChecker* cert_checker_;