Reset external call values after listener is removed

External CallList and CallNotifier values not resetting
after listener is removed. Crash is observing on clicking
notification bar.

Change-Id: Ib8da5dece91cbc98a25940ba74720f2642b1af5a
CRs-Fixed: 1107363
diff --git a/InCallUI/src/com/android/incallui/InCallPresenter.java b/InCallUI/src/com/android/incallui/InCallPresenter.java
index 4b7f311..59c39a3 100644
--- a/InCallUI/src/com/android/incallui/InCallPresenter.java
+++ b/InCallUI/src/com/android/incallui/InCallPresenter.java
@@ -1710,6 +1710,9 @@
             if (mExternalCallNotifier != null && mExternalCallList != null) {
                 mExternalCallList.removeExternalCallListener(mExternalCallNotifier);
             }
+            mExternalCallNotifier = null;
+            mExternalCallList = null;
+
             mStatusBarNotifier = null;
 
             InCallCsRedialHandler.getInstance().tearDown();