Log screen-on latency with tron

Bug: 36776886
Test: manual test with "adb logcat -b all" and systrace.
Change-Id: I9e7188475a79899569046892e6f08c9005b84303

core/java/android/metrics/LogMaker.java

@@ -93,6 +93,16 @@ public class LogMaker {
         return this;
     }
 
+    /**
+     * Set event latency.
+     *
+     * @hide // TODO Expose in the future?  Too late for O.
+     */
+    public LogMaker setLatency(long milliseconds) {
+        entries.put(MetricsEvent.NOTIFICATION_SINCE_CREATE_MILLIS, milliseconds);
+        return this;
+    }
+
     /**
      * This will be set by the system when the log is persisted.
      * Client-supplied values will be ignored.