Revert "@hide ConnectionService APIs."

This reverts commit 86935c406e4a8bb0e2ad871b51009f3603df9e87.

telecomm/java/android/telecom/Conference.java

@@ -16,8 +16,6 @@
 
 package android.telecom;
 
-import android.annotation.SystemApi;
-
 import java.util.Collections;
 import java.util.List;
 import java.util.Set;
@@ -26,9 +24,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
 /**
  * Represents a conference call which can contain any number of {@link Connection} objects.
- * @hide
  */
-@SystemApi
 public abstract class Conference {
 
     /** @hide */