Hide Adhoc conference call API.

Marking the adhoc conference call API as @hide.

Test: make -j update-api
Bug: 148165780
Change-Id: I447ec317c66f0c1b4e7a828635b5dcf8b144e32a

telecomm/java/android/telecom/Connection.java

@@ -501,7 +501,7 @@ public abstract class Connection extends Conferenceable {
      * Set by the framework to indicate that it is an adhoc conference call.
      * <p>
      * This is used for Outgoing and incoming conference calls.
-     *
+     * @hide
      */
     public static final int PROPERTY_IS_ADHOC_CONFERENCE = 1 << 12;