InCallActivity: consume back key on incoming call screen

Users will not be able to anything if the physical back key is
pressed on the incoming call screen (return to home screen with
nothing responding.)

Forward porting the key consumption from 4.3

Change-Id: Iea0734852bac7ca5a25e19ec9d9ee7c80dba6741
diff --git a/src/com/android/incallui/ b/src/com/android/incallui/
index 08a8f12..3d4d6f3 100644
--- a/src/com/android/incallui/
+++ b/src/com/android/incallui/
@@ -192,6 +192,17 @@
     public void onBackPressed() {
         Log.d(this, "onBackPressed()...");
+        if (mAnswerFragment.isVisible()) {
+            // The Back key, just like the Home key, is always disabled
+            // while an incoming call is ringing.  (The user *must* either
+            // answer or reject the call before leaving the incoming-call
+            // screen.)
+            Log.d(this, "BACK key while ringing: ignored");
+            // And consume this event; *don't* call super.onBackPressed().
+            return;
+        }
         // BACK is also used to exit out of any "special modes" of the
         // in-call UI: