Increment a MetricsLogger counter on back press.

am: 580135c1b8

Change-Id: I29fb7f03b5da8edf3643118bc74bba553f2f6ca1

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;