IMS: Disconnect children before ending conference call

Disconnect the children of conference call before
ending the conferece call itself.

Bug: 24677801
Change-Id: Ia168360926b87a2482c607b6fe7c314faae9c184
diff --git a/src/com/android/services/telephony/ImsConference.java b/src/com/android/services/telephony/ImsConference.java
index c7c6e92..8ccc8f9 100644
--- a/src/com/android/services/telephony/ImsConference.java
+++ b/src/com/android/services/telephony/ImsConference.java
@@ -824,6 +824,7 @@
                             mConferenceHost.getOriginalConnection().getDisconnectCause());
                 }
                 setDisconnected(disconnectCause);
+                disconnectConferenceParticipants();
                 destroy();
                 break;
             case Connection.STATE_ACTIVE: