Make Conferenceable an abstract class

Bug: 21066017
Change-Id: Ibcbccfc7b1380e417b464aed9b968cbc0a6a1cc2
diff --git a/telecomm/java/android/telecom/Conferenceable.java b/telecomm/java/android/telecom/Conferenceable.java
index 5c4cbc5..bb6f2b8 100644
--- a/telecomm/java/android/telecom/Conferenceable.java
+++ b/telecomm/java/android/telecom/Conferenceable.java
@@ -21,6 +21,6 @@
  * call with.  The {@link ConnectionService} implementation will only recognize
  * {@link Conferenceable}s which are {@link Connection}s or {@link Conference}s.
  */
-public interface Conferenceable {
-
+public abstract class Conferenceable {
+    Conferenceable() {}
 }