Build update_engine_sideload.

Add a new "sideload" executable target that applies an update payload
directly using the UpdateAttempterAndroid.

This initial CL buils a dynamically linked program that's targeted to
run in the system image for now, but will later be transformed into a
static binary to run from the recovery environment.

Bug: 27178350
TEST=Applied a payload directly on a device using:
`update_engine_sideload --payload=file://foo/bar ...`

Change-Id: I289a724d013abdc390187d669dccd3edf2fd3434
diff --git a/daemon_state_interface.h b/daemon_state_interface.h
index a0944aa..2356816 100644
--- a/daemon_state_interface.h
+++ b/daemon_state_interface.h
@@ -20,6 +20,7 @@
 #include "update_engine/service_observer_interface.h"
 
 #include <memory>
+#include <set>
 
 namespace chromeos_update_engine {
 
@@ -36,6 +37,9 @@
   virtual void AddObserver(ServiceObserverInterface* observer) = 0;
   virtual void RemoveObserver(ServiceObserverInterface* observer) = 0;
 
+  // Return the set of current observers.
+  virtual const std::set<ServiceObserverInterface*>& service_observers() = 0;
+
  protected:
   DaemonStateInterface() = default;
 };