Fix ClassCastException for ImsExternalConnection

A Connection may be a ImsExternalConnection, not ImsPhoneConnection, if
the phone type is IMS. Add check to remove the ClassCastException.

Bug: 74375015
Test: manual
Change-Id: Ibb116fc2e8a7667adb19b61f053207d28a4582a5
diff --git a/src/com/android/services/telephony/TelephonyConnection.java b/src/com/android/services/telephony/TelephonyConnection.java
index 0fe6980..af9e9d5 100644
--- a/src/com/android/services/telephony/TelephonyConnection.java
+++ b/src/com/android/services/telephony/TelephonyConnection.java
@@ -1698,6 +1698,7 @@
     private boolean isRtt() {
         return mOriginalConnection != null
                 && mOriginalConnection.getPhoneType() == PhoneConstants.PHONE_TYPE_IMS
+                && mOriginalConnection instanceof ImsPhoneConnection
                 && ((ImsPhoneConnection) mOriginalConnection).isRttEnabledForCall();
     }