Fix to answer call in DSDS mode.

Reset the mCall reference to null when a call is disconnected

Change-Id: I91d58519ea504e3a031d6c2f706b98e21e4270fa
CRs-Fixed: 945977
diff --git a/src/com/android/incallui/AnswerPresenter.java b/src/com/android/incallui/AnswerPresenter.java
index 732fe06..814dcf7 100644
--- a/src/com/android/incallui/AnswerPresenter.java
+++ b/src/com/android/incallui/AnswerPresenter.java
@@ -273,7 +273,9 @@
 
     @Override
     public void onDisconnect(Call call) {
-        // no-op
+        int subId = call.getSubId();
+        int phoneId = mCalls.getPhoneId(subId);
+        mCall[phoneId] = null;
     }
 
     public void onSessionModificationStateChange(int sessionModificationState) {