Converted UiSharedTopologyModel to be a @Service.

Change-Id: Idc8df1b9c0a52db01ac545567dacc0e1c770f84a
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
index e54ac66..5f4195e 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
@@ -19,5 +19,5 @@
 /**
  * Represents the overall network topology.
  */
-public class UiTopology {
+public class UiTopology extends UiElement {
 }