Fix crash on shutdown when update in progress.

The ActionProcessor destructor normally calls the ProcessingStop method
on the delegate. For the UpdateAttempter this call re-schedules a new
update attempt on a half-destroyed update_attempter instance, crashing
update_engine on SIGTERM when the ActionProcessor was running.

This patch inhibits the ActionProcessor from notifying the delegate of
the processor stopping when destroying the update_attempter instance.
It also fixes the declaration order of the dbus_adaptor_ and disables
its usage during daemon shutdown.

Bug: 24989397
TEST=start update-engine; (update_engine_client --update &); sleep 6; stop update-engine; tail /var/log/update_engine.log
TEST=FEATURES=test emerge-link update_engine

Change-Id: I0a40067f63e89759ff80c79cecb6f89b10dba0c2
diff --git a/daemon.cc b/daemon.cc
index 0830feb..0b13c18 100644
--- a/daemon.cc
+++ b/daemon.cc
@@ -54,6 +54,14 @@
 }
 }  // namespace
 
+UpdateEngineDaemon::~UpdateEngineDaemon() {
+  UpdateAttempter* update_attempter = real_system_state_->update_attempter();
+  // Prevent any DBus communication from UpdateAttempter when shutting down the
+  // daemon.
+  if (update_attempter)
+    update_attempter->set_dbus_adaptor(nullptr);
+}
+
 int UpdateEngineDaemon::OnInit() {
   // Register the |subprocess_| singleton with this Daemon as the signal
   // handler.