Make Conferenceable an abstract class

Bug: 21066017
Change-Id: Ibcbccfc7b1380e417b464aed9b968cbc0a6a1cc2
diff --git a/telecomm/java/android/telecom/Conference.java b/telecomm/java/android/telecom/Conference.java
index d8d9ab6..dfbb67a 100644
--- a/telecomm/java/android/telecom/Conference.java
+++ b/telecomm/java/android/telecom/Conference.java
@@ -30,7 +30,7 @@
 /**
  * Represents a conference call which can contain any number of {@link Connection} objects.
  */
-public abstract class Conference implements Conferenceable {
+public abstract class Conference extends Conferenceable {
 
     /**
      * Used to indicate that the conference connection time is not specified.  If not specified,