Android: Implement the binder service.

This patch introduces the service delegate interface for non-Brillo
targets, which is the class in charge of implementing the API exposed
by the service. The binder service for non-Brillo targets is now
completed using this interface.

The other side of this interface will be implemented in a follow up CL,
while this CL includes only the interface and its usage.

To accomodate non-Brillo targets, the generic ServiceObserverInterface
is extended with the PayloadApplicationComplete message, which will be
implemented for all targets in the future.

Bug: 25631949
TEST=`mmma system/update_engine` on aosp_arm-eng and edison-eng

Change-Id: I9fa8e9565ae92515e81e07d2cef562fc4e11a7ba
diff --git a/daemon_state_android.cc b/daemon_state_android.cc
index 3e89ad0..c1d9fcc 100644
--- a/daemon_state_android.cc
+++ b/daemon_state_android.cc
@@ -35,4 +35,9 @@
   service_observers_.erase(observer);
 }
 
+ServiceDelegateAndroidInterface* DaemonStateAndroid::service_delegate() {
+  // TODO(deymo): Implement a service delegate and return it here.
+  return nullptr;
+}
+
 }  // namespace chromeos_update_engine