API declarations for Call Handover.

Design doc:
https://docs.google.com/document/d/1qY3oAzjff_4A1ttYb_CGrE_OwTRmXMG_KGsIuPT1ey8/edit#

Bug: 65415068
Test: manual.
Change-Id: I0c2f561d92ad6504f858eadde09980fc1ce8727f

telecomm/java/android/telecom/ConnectionService.java

@@ -2032,6 +2032,43 @@ public abstract class ConnectionService extends Service {
         return null;
     }
 
+    /**
+     * Called by Telecom on the initiating side of the handover to create an instance of a
+     * handover connection.
+     * @param fromPhoneAccountHandle {@link PhoneAccountHandle} associated with the
+     *                               ConnectionService which needs to handover the call.
+     * @param request Details about the call which needs to be handover.
+     * @return Connection object corresponding to the handover call.
+     */
+    public Connection onCreateOutgoingHandoverConnection(PhoneAccountHandle fromPhoneAccountHandle,
+                                                         ConnectionRequest request) {
+        return null;
+    }
+
+    /**
+     * Called by Telecom on the receiving side of the handover to request the
+     * {@link ConnectionService} to create an instance of a handover connection.
+     * @param fromPhoneAccountHandle {@link PhoneAccountHandle} associated with the
+     *                               ConnectionService which needs to handover the call.
+     * @param request Details about the call which needs to be handover.
+     * @return {@link Connection} object corresponding to the handover call.
+     */
+    public Connection onCreateIncomingHandoverConnection(PhoneAccountHandle fromPhoneAccountHandle,
+                                                         ConnectionRequest request) {
+        return null;
+    }
+
+    /**
+     * Called by Telecom in response to a {@code TelecomManager#acceptHandover()}
+     * invocation which failed.
+     * @param request Details about the call which needs to be handover.
+     * @param error Reason for handover failure as defined in
+     *              {@link android.telecom.Call.Callback#HANDOVER_FAILURE_DEST_INVALID_PERM}
+     */
+    public void onHandoverFailed(ConnectionRequest request, int error) {
+        return;
+    }
+
     /**
      * Create a {@code Connection} for a new unknown call. An unknown call is a call originating
      * from the ConnectionService that was neither a user-initiated outgoing call, nor an incoming