Increment a MetricsLogger counter on back press. am: 580135c1b8

am: 51ac3a911c

Change-Id: I0b65ca68a743828fa47efd6793c5873cbc9b74a9

services/core/java/com/android/server/policy/PhoneWindowManager.java

@@ -1042,6 +1042,7 @@ public class PhoneWindowManager implements WindowManagerPolicy {
     }
 
     private void interceptBackKeyDown() {
+        MetricsLogger.count(mContext, "key_back_down", 1);
         // Reset back key state for long press
         mBackKeyHandled = false;