Apply @hide / @SystemApi to android.telecom.*

Move the android.telecom.* namespace back to @hide, and also mark it
with @SystemApi so that system-privileged apps can use them.

Bug: 18302450
Change-Id: I33ae1b9b0dfdb1c5eff51ca3c829196bcfc9411c

telecomm/java/android/telecom/Conference.java

@@ -16,6 +16,8 @@
 
 package android.telecom;
 
+import android.annotation.SystemApi;
+
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -25,7 +27,9 @@ 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 */